Chemistry data questions
Octane and isooctane can be separated in the laboratory. Name a laboratory technique that could be used to separate isooctane from a mixture of octane and isooctane. Outline how this technique separates isooctane from octane.
(fractional or simple) distillation. Isooctane (the one with the lower boiling point) boils (first). Isooctane condenses and collected away from the octane.
Sodium hydroxide is often sold as a concentrated solution containing 12.0 mol dm-3 of sodium hydroxide.Calculate the volume of water that should be added to 10.0 cm3 of a 12.0 mol dm-3 solution of sodium hydroxide to make a 0.250 mol dm-3 solution. Show your working.
12 divided by 0.25 equals 48 48 multiplied by 10 equals 480 480 ,minus 10 equals 470 cm3
What is a burette?
A burette is a graduated glass tube with a tap at one end, for delivering known volumes of a liquid, especially in titrations.
Purpose of a burette
A burette is used to deliver solution in precisely-measured, variable volumes. Burets are used primarily for titration, to deliver one reactant until the precise end point of the reaction is reached.
What is a pipette used for?
A pipette is used to measure small amounts of solution very accurately. A pipette bulb is used to draw solution into the pipet.
In Part 1 of Test 3, sodium hydroxide was added to a solution of iron(II) ions. In Part 2 of Test 3, the mixture was shaken in air. State what you would have observed if an excess of hydrochloric acid had been added to the final mixture in the test tube.
A solution would form.
What is a volumetric flask used for?
A volumetric flask is used to make up a solution of fixed volume very accurately. This volumetric flask measures 500 mL ± 0.2 mL.To make up a solution, first dissolve the solid material completely, in less water than required to fill the flask to the mark.
A student repeated the experiment but filled the volumetric flask significantly above the graduation mark. The student then completed the rest of the task correctly. Explain why this error results in a higher calculated value of Kc
Acid in the flask would be more dilute. It would appear that more acid had turned into ester
State some potential hazards in the experiment and the relevant safety precaution you would take
Acid is corrosive so wear eye protection/gloves. Carbonate is toxic so wear gloves.
Sodium hydrogencarbonate in baking powder forms carbon dioxide during the production of bread and cakes. Suggest one advantage of having an acid in baking powder.
Acid reacts. Acid neutralises sodium hydrogen carbonate to form carbon dioxide.
Identify a reagent that could be used to show that solution X is not contaminated with iodide ions. Describe what you would observe.
Add acidifed silver nitrate solution. No visible change.
Percentage uncertainties are combined using certain rules. Whats the rule for adding or subtracting values?
Add the absolute uncertainties (a=b+c)
Percentage uncertainties are combined using certain rules. What's the rule for multiplying values?
Add the percentage uncertainties
A sample of sodium carbonate was thought to be the monohydrate, Na2CO3.H2O. A student was asked to confirm the identity of the sample by titration against hydrochloric acid.State two ways of improving the student's method of weighing the sodium carbonate. In each case explain why the accuracy of the experiment would be improved.
Add washings from bottle so that all the sample is transferred to the flask. Use a 3 dp balance to reduce error in weighing.
A strong alkali such as potassium hydroxide is used as the electrolyte in some alkaline batteries for household use. The electrolyte will escape if the battery casing is broken. Suggest one reason why a leak of this electrolyte is hazardous.
Alkali is corrosive / caustic. It can burn skin.
A salt bridge is used to complete the cell. This could be prepared using potassium nitrate solution and filter paper. State the purpose of the salt bridge. State one essential requirement of the soluble ionic compound used to make the salt bridge.
Allows for the movement of ions between electrodes to maintain an electric current. It must not react with the ions in solution
State two properties that a liquid must have to make it suitable for use in the heating bath in this boiling point determination.
Boiling point above that of sample. Does not ignite easily (not flammable)
State why the atmospheric pressure was recorded.
Boiling point varies with pressure
At a given pressure, a pure liquid boils at a constant temperature. This boiling point can be used to help identify the liquid. One procedure for measuring the boiling point of an aldehyde is shown in the diagram.Give one reason why it is essential for the cork to have the hole in it.
By allowing the vapour to escape the hole prevents the pressure building up and so prevents an explosion.
Write an equation to show the reaction of sodium hydroxide with carbon dioxide
CO2 (g) + H2O (l)→ H2CO3 (aq) H2CO3 (aq) + NaOH (aq) →NaCO3 (aq) + H2O (l)
Calorimetry
Calorimetry is used to determine the heat released or absorbed in a chemical reaction. Key techniques for obtaining accurate results are starting with a dry calorimeter, measuring solution volumes precisely, and determining change in temperature accurately.
Use information from the diagram to suggest why ammonia is not regarded as a raw material in the Solvay Process.
Can act as a catalyst. because it is used up but then regenerated later in the cycle.
Sodium carbonate is manufactured by the Solvay Process.In Reactor 1, calcium carbonate is decomposed into calcium oxide and carbon dioxide. Despite no significant leakage of carbon dioxide from this decomposition, this part of the process results in an increase in carbon dioxide in the atmosphere. State why this increase in carbon dioxide occurs.
Carbon dioxide from burning fossil fuels.
Suggest with a reason an appropriate safety precaution that the student should take when using the toxic chloralkane, X in the experiment.
Carry out in a fume cupboard to avoid toxic vapour.
Explain how infrared spectroscopy can be used to show that an aldehyde is definitely pentanal.
Compare spectrum of aldehyde with known spectrum of pentanal. fingerprint regions match exactly.
Suggest one reason why Tollens' reagent is used as the oxidising agent in the specific test for aldehydes rather than the less expensive acidified potassium dichromate(VI).
Dichromate(VI) will also oxidise alcohols and so will give a positive test. Dichromate(VI) will oxidise other functional groups such as aldehydes.
In a series of titrations using the solution made up in Question 17(a), a student obtained the following titres .State what this student must do in order to obtain an accurate average titre in this experiment.
Do further titrations o obtain concordant results ( results that have a 0.1 difference).
Other than cost, explain one advantage of using magnesium hydroxide rather than magnesium carbonate to reduce acidity in the stomach.
Does not produce carbon dioxide gas which distends stomach ( does not increase pressure in stomach).
Explain why it was necessary for the student to heat the crucible and its contents to constant mass.
Ensure reaction complete. Ensures all water lost by evaporation.
What are conical flasks and beaker used for?
Erlenmeyer flasks and beakers are used for mixing, transporting, and reacting, but not for accurate measurements. The volumes stamped on the sides are approximate and accurate to within about 5%.
Because of their many uses, phthalates have been tested for possible adverse effects to humans and to the environment. The text below is taken from a document written by the organisation. 'Research demonstrates that phthalates, at current and foreseeable exposure levels, do not pose a risk to human health or to the environment. Suggest what needs to be done so that the public could feel confident that the research quoted above is reliable.
Experiment continued over a long period. Experiment repeated by other scientists to avoid bias. Investigate breakdown products. Experiment results should be made public
Dilute solutions of sodium carbonate and hydrochloric acid are both classified as irritants. State the essential safety precaution to be taken when using these irritant.
Eye protection must be worn
Succinic acid can be produced by the fermentation of glucose.Suggest and explain the main environmental impact of fermentation reactions.
Fermentation reaction produces carbon dioxide and carbon dioxide causes global warming.
Anti-bumping granules are placed in the flask when refluxing.Suggest why these granules prevent bumping.
Form smaller bubbles. (prevent large bubbles)
What are Graduated cylinders useful for?
Graduated cylinders are useful for measuring liquid volumes to within about 1%. They are for general purpose use, but not for quantitative analysis. If greater accuracy is needed, use a pipet or volumetric flask.
When the student had finished the investigation, an excess of sodium hydroxide solution was added to the reaction mixture. This was to neutralise any unreacted sulfuric acid. The student found that a further reaction took place, producing magnesium hydroxide. Draw a diagram to show how the student could separate the magnesium hydroxide from the reaction mixture.
Gravity: Conical flask or and funnel. Vacuum: Sealed container with a side arm and Buchner funnel. Filter paper
A student carried out an experiment to determine the enthalpy of combustion of methanol.Suggest three reasons, other than incomplete combustion or heat transfer to the atmosphere, why the student's value for the enthalpy of combustion of methanol is different from that in a Data Book.
Heat capacity of copper not taken into account. Evaporation of alcohol. Experiment not done under standard conditions.
A student carried out an experiment to determine the enthalpy of combustion of methanol. The student said correctly that using a thermometer with an overall uncertainty for the rise in temperature of ±0.5 °C was adequate for this experiment. Explain why this thermometer was adequate for this experiment.
Heat loss is more significant issue. Temperature change significantly bigger than uncertainty.
In calorimetry identify the main source of error and suggest one improvement to minimise this main source of error. Identify another source of error.
Heat loss is the main source of error. To minimse this error you could add a lid to the colorimeter to provide insulation. Temperature rise to small. This can be adjusted by adding a higher concnetration of reagent.
During the preparation of aspirin, it is necessary to filter the crude product under reduced pressure.You are provided with a small sample of pure aspirin in a melting point tube. Describe briefly how you would determine an accurate value for the melting point of aspirin.
Heat melting point tube (apparatus or Thiele tube) in an oil bath slowly near the melting point.
A student carried out Task 1 on a different pure sample of sodium hydrogencarbonate. In this case, the mass of solid in the crucible at the end of the experiment was 87.3% of the original mass. Compare this percentage with your answer to Question 2 and suggest an experimental reason for this student's result
If the answer was higher say crucible was not heated long enough (incomplete decomposition). If answer was lower say further decomposition took place.
A group of six students carried out this experiment. They each weighed out a sample of hydrated sodium carbonate. They then heated their sample to a temperature higher than 300 ºC in a crucible for ten minutes and recorded the final mass after the crucible had cooled.All the students carried out the experiment exactly according to this method. Explain why a student that you identified in Question 8 (c) obtained an anomalous result.
Incompletete decomposition.
How could you reduce the percentage error?
Increase the amount of the solution used. % error = (uncertainty in equipment / reading) x 100. Uncertainty in the equipment remains constant, but if you have a larger reading (more solution) then you will have a smaller % error.
Safety information indicates that tartaric acid and its salts can act as muscle toxins. These can cause paralysis and possible death. Suggest one reason why the use of tartaric acid in baking powder is not a hazard to health.
It is only used in very small quantities. It decomposes in the reaction
Suggest one reason why a 250 cm3 conical flask is preferred to a 250 cm3 beaker for a titration.
Less chance of losing liquid on swirling. Liquid doesn't splash on swirling.
similar titration was carried out with a different sample of pure washing soda that had been stored for some time. A student obtained a value of 8.6 for the value of x.Assume that the student carried out the titration and the calculation accurately. State one reason why the number of moles of water of crystallisation is less than 10.
Loss of water
Pure magnesium reacts completely with an excess of dilute sulfuric acid. The reaction of pure calcium with an excess of dilute sulfuric acid is very rapid initially. This reaction slows down and stops before all of the calcium has reacted. Use your knowledge of the solubilities of Group 2 sulfates to explain why these reactions of magnesium and calcium with dilute sulfuric acid are so different.
Magnesium sulfate is soluble and calcium sulfate is insoluble. Calcium sulfate coats the surface of the calcium which prevents further contact with acid.
Explain briefly what you could do to confirm that 1 week was sufficient time for equilibrium to be established in the mixture from Part 1.
Make up more than one mixture and test one mixture after 1 week and another mixture after a longer time. Titre values should be the same if equilibrium has been established
State why the boiling point of a compound may be insufficient on its own to identify the compound.
Many compounds have the same boiling point.
A student investigated how the initial rate of reaction between sulfuric acid and magnesium at 20 ºC is affected by the concentration of the acid.The student made measurements every 20 seconds for 5 minutes. The student then repeated the experiment using double the concentration of sulfuric acid. State one condition, other than temperature and pressure, that would need to be kept constant in this investigation.
Mass of magnesium
Ammonia and methylamine were dissolved in separate samples of water. The two solutions had equal molar concentrations. State one simple method, other than smell, of distinguishing these solutions. State what you would observe.
Measure pH with a meter. Methylamine would have a higher pH.
A student investigated how the initial rate of reaction between sulfuric acid and magnesium at 20 ºC is affected by the concentration of the acid.The student made measurements every 20 seconds for 5 minutes. The student then repeated the experiment using double the concentration of sulfuric acid.State a measurement that the student should make every 20 seconds. Identify the apparatus that the student could use to make this measurement,
Measure the volume of gas using a gas syringe.
In order to obtain a pH curve, you are provided with a conical flask containing 25.0cm3 of a 0.100moldm-3 carboxylic acid solution and a burette filled with 0.100moldm-3 sodium hydroxide solution. You are also provided with a calibrated pH meter.State why calibrating a pH meter just before it is used improves the accuracy of the pH measurement.
Over time (after storage) meter does not give accurate readings. Temperature variations affect readings.
Explain how you could use your results from the experiment in Question 9 to determine the initial rate of this reaction.
Plot a graph of 'volume (of gas)' against 'time' Determine the slope (gradient) at the beginning.
Explain how you would accurately determine the change in temperature in a calorimetry experiment once you have obtained your results.
Plot a graph of temperature vs time. Extrapolate back to time of mixing = 0 and determine the temperature.
Sodium hydroxide is used to remove grease from metal components. Sodium hydroxide cannot be used to clean components made of aluminium because it reacts with this metal In 1986, a sealed aluminium tank exploded while being used by mistake for transporting concentrated sodium hydroxide solution. Suggest one reason why the tank exploded.
Pressure build-up due to the production of hydrogen gas
Copper, in the form of nanoparticles of copper(II) hexacyanoferrate(II), has recently been investigated as an efficient method of storing electrical energy in a rechargeable cell. Suggest one reason why many waste disposal centres contain a separate section for cells and batteries.
Prevent pollution of the environment by toxic substances. Enables recycling of valuable components.
Suggest one reason why placing a lid on the crucible might increase the accuracy of the experiment.
Prevents loss of solid during heating and allows only gas to escape.
State the purpose of anti-bumping granules.
Prevents vigorous boiling/ bubbling
The total error in a titre is caused by three factors. Name them
Reading the burette at the start of the titration. Reading the burette at the end of the titration. Judging the end point to within one drop.
Difference between measurements and readings
Readings require one judgment only, measurements require two.
A student carried out an experiment on a pure sample of 2-methylpropan-2-ol to determine its enthalpy of combustion. A sample of the alcohol was placed into a spirit burner and positioned under a beaker containing 50 cm3 of water. The spirit burner was ignited and allowed to burn for several minutes before it was extinguished.Suggest one improvement that would reduce errors due to heat loss in the student's experiment.
Reduce the distance between the flame and the beaker.Put a sleeve around the flame to protect from drafts.Use a copper calorimeter rather than a pyrex beaker. Calibrate the equipment using an alcohol of known enthalpy of combustion.
Triiodomethane can be separated from the reaction mixture by filtration. State one reason why the solid residue is then washed with water after the filtration.
Remove soluble impurities ( or excess reagents)
During a titration, a chemist rinsed the inside of the conical flask with deionised water. The water used for rinsing remained in the conical flask.Give one reason why this rinsing can improve the accuracy of the end-point.
Returns reagent on the sides of the flask to the reaction mixture to ensure that all of the acid / alkali reacts.
The rate of the oxidation of iodide ions to iodine by peroxodisulfate(VI) ions (S2O82-) was studied by measuring the concentration of the l3- ions at different times, starting at time = 0, when the reactants were mixed together. The concentration of the l3- ions was determined by measuring the absorption of light using a spectrometer. A graph of these results should include an additional point. On the grid, draw a ring around this additional point.
Ring around the origin.
Suggest one reason why the student should not stir the barium bromide during heating.
Salt will adhere to stirrer. Mass will be lost.
Magnesium ribbon reacts with hot water. Heated magnesium ribbon reacts with steam. State three differences between these reactions.
Slower with hot water and faster with steam .he hot water produces magnesium hydroxide and the steam produces magnesium oxide.Slow bubbling with hot water and bright white light with white solid with steam.
A student carried out a titration by adding an aqueous solution of sodium hydroxide from a burette to an aqueous solution of ethanoic acid.Suggest why the concentration of sodium hydroxide in a solution slowly decreases when left open to air.
Sodium hydroxide reacts with carbon dioxide in the air
Copper, in the form of nanoparticles of copper(II) hexacyanoferrate(II), has recently been investigated as an efficient method of storing electrical energy in a rechargeable cell.Solar cells generate an electric current from sunlight. These cells are often used to provide electrical energy for illuminated road signs. Explain why rechargeable cells are connected to these solar cells
Solar cells do not supply electrical energy all the time. Rechargeable cells can store electrical energy for use when the solar cells are not working.
Calorimetry steps
Solutions volumes should be carefully measured with a graduated cylinder. Add solution completely, to a dry calorimeter. Don't forget to add the spin bar each time. Set up the calorimeter with the thermometer (0° to 50°C, graduated every 0.1°C) supported from a stand so that the bulb does not touch the bottom of the cup. Note that the thermometer used for calorimetry differs from the less accurate one in your glassware drawer. Clamp the calorimeter so that it rests on the stirrer. Be careful not to turn on the heat or you will melt the styrofoam.
In this experiment, no side reactions take place, the reagents are pure and the reaction goes to completion. Suggest one reason why the yield of iron(II) ethanedioate dihydrate in this experiment is less than 100%.
Some left in solution / some lost during filtration
The correct technique can improve the accuracy of a titration. State why it is important to fill the space below the tap in the burette with solution A before beginning an accurate titration.
Space will fill during titration. Titres or volumes added are too high.
This question is about reactions of calcium compounds. A pure solid is thought to be calcium hydroxide. The solid can be identified from its relative formula mass. The relative formula mass can be determined experimentally by reacting a measured mass of the pure solid with an excess of hydrochloric acid. The unreacted acid can then be determined by titration with a standard sodium hydroxide solution.You are provided with 50.0 cm3 of 0.200 mol dm-3 hydrochloric acid. Outline, giving brief practical details, how you would conduct an experiment to calculate accurately the relative formula mass of the solid using this method. Ca(OH)₂ + 2HCl CaCl₂ + 2H₂O
Statement that there must be an excess of acid Moles of acid = 50.0 × 0.200/1000 = 1.00 × 10⁻² mol. 2 mol of acid react with 1 mol of calcium hydroxide therefore moles of solid weighed out must be less than half the moles of acid: 0.5 × 1.00 × 10-2 = 5.00 × 10-3 mol Mass of solid must be < 5.00 × 10-3 × 74.1 = < 0.371 g Measure out 50 cm3 of acid using a pipette and add the weighed amount of solid in a conical flask Titrate against 0.100 (or 0.200) mol dm-3 NaOH added from a burette and record the volume (v) when an added indicator changes colour. How to calculate Mr from the experimental data. Moles of calcium hydroxide= 5.00 × 10-3 - (v/2 × conc NaOH)/1000 = z mol Mr = mass of solid / z
The student identified use of the burette as the largest source of uncertainty in the experiment. Using the same apparatus, suggest how the procedure could be improved to reduce the percentage uncertainty in using the burette. Justify your suggested improvement.
Suggestion: Use a larger mass of solid or use a more concentrated solution of the metal carbonate or a less concentrated solution of HCl. Justification: So a larger titre will be needed. So a larger volume of HCl will be needed.
In each titration, the volume of sulfuric acid was measured approximately with a measuring cylinder.Suggest why this approximate measure did not affect the accuracy of the titre.
Sulfuric acid is in excess. sulfuric acid not used in titration calculation.
In an experiment to determine the acid dissociation constant (Ka) of a weak acid, 25.0 cm3 of an approximately 0.1 mol dm-3 solution of this acid were titrated with a 0.10 mol dm-3 solution of sodium hydroxide. The pH was measured at intervals and recorded. Suggest how the experimental procedure could be slightly modified in order to give a more reliable value for the end-point.
Take more pH readings around the end-point or add smaller volumes of NaOH near the end-point
In Test 4 of Task 2, suggest why there was no visible change when the first few drops of dilute ammonia solution were added to the filtrate.
The ammonia is initially neutralised with the excess sulfuric acid.
Deduce two reasons why the hazard associated with the formation of sulfur dioxide in this investigation might be considered to be low.
The amount of sulfur dioxide formed in each experiment is small. The amount released into the air is small. The solutions are dilute. The reaction has not gone to completion. The solutions were disposed of correctly. The sulfur dioxide is soluble.
A simple salt bridge can be prepared by dipping a piece of filter paper into potassium carbonate solution. Explain why such a salt bridge would not be suitable for use in this cell.
The carbonate ion reacts with the acid in the standard hydrogen electrode. H+/ Al³⁺ concentrations change.
In calorimetry how is ∆T determined?
The change in temperature is determined by measuring the initial temperature (T1) of the reactants, and the maximum temperature (T2) of the contents of the calorimeter during the exothermic reaction. Use a magnifying glass to measure temperature values precisely.
The magnesium used in a laboratory experiment was supplied as a ribbon. The ribbon was stored in an open plastic bag exposed to the air. Explain why it is important to clean the surface of this magnesium ribbon when investigating the rate of its reaction with hydrochloric acid.
The magnesium is coated with an oxide (MgO). The coating has to be removed before Mg will react.
Suggest why the pink colour of the phenolphthalein fades after the end-point of the titration has been reached. Ignore any absorption of acidic gases from the atmosphere.
The mixture is turning more acidic as the ester is hydrolysed. Position of equilibrium moves to the left
State what could be done to improve the percentage atom economy of the Solvay Process.
The percentage atom economy cannot be improved. OR Sell the by-product.
What are measurments?
The values taken as the difference between the judgements of two values
Baking powder usually contains starch. Starch is added to absorb any water vapour that may come into contact with the baking powder when the container is opened. Deduce a reason why this water vapour needs to be absorbed.
The water would allow the tartaric acid and sodium hydrogencarbonate to react before use
In a titration, it is important to wash the inside of the titration flask with distilled or deionised water as you approach the end-point. Suggest one reason why it is important to wash the inside of the flask.
To make sure all the solutions from both the burette and pipette react with each other.
Suggest one reason why repeating a titration makes the value of the average titre more reliable
To obtain concordant results as a single titration could be flawed (anomalous).
The reaction between zinc and hydrochloric acid in Part 2 of Test 1 produces hydrogen in the boiling tube. This gas helps to provide the conditions needed for the formation of the final vanadium-containing product. Suggest why a cotton wool plug is used in this tube and why a rubber bung is less suitable.
To prevent oxygen entering the tube and to keep the hydrogen gas in the test tube. Pressure would build up in the tube if it was sealed with a rubber bung.
The aluminium used as the electrode is rubbed with sandpaper prior to use. Suggest the reason for this
To remove the oxide layer on the aluminium
Substances that contain carbonate or hydrogencarbonate ions can be used to confirm the presence of an acid. Identify one other substance that could be used to confirm the presence of acid groups in tartaric acid. State the observation you would make when this other substance is added to an aqueous solution of tartaric acid.
Universal Indicator (litmus paper). Colour turns red. OR. Add a reactive metal such as magnesium.Gas evolved.
Suggest a suitable titre for the titration. Explain your reason for choosing this titre.
Use 30 cm cubed because this titre will give a low burette percentage error. Low volumes gives smaller mass and increase errors in weighing mass.
Apart from loss of liquid during transfer to the gas syringe, identify one other source of error in this experiment. Suggest one improvement to minimise this other source of error
Use a 3 fig balance for more precision. Maintain constant temperature better by preventing heat loss from oven by better using a steam jacket for better insulation.
An accurate value for the enthalpy of combustion of 2-methylpropan-2-ol in which water is formed as a gas is -2422 kJ mol-1.Use this value and your answer from Question 8 (b) to calculate the overall percentage error in the student's experimental value for the enthalpy of combustion of 2-methylpropan-2-ol.
Use percentage change calculation
Succinic acid is toxic and corrosive but is used in the food industry as a sweetener. Explain why this use of succinic acid is acceptable.
Used in small amounts
What is vaccumm titration
Vacuum filtration is a technique for separating a solid product from a solvent or liquid reaction mixture. The mixture of solid and liquid is poured through a filter paper in a Buchner funnel. The solid is trapped by the filter and the liquid is drawn through the funnel into the flask below, by a vacuum.
The following instructions describe how to make up a standard solution of a solid in a volumetric flask. 'Take a clean 250cm3 volumetric flask. Use the balance provided and a clean, dry container, to weigh out the amount of solid required. Tip the solid into a clean, dry 250cm3 beaker and add about 100cm3 of distilled water. Use a stirring rod to help the solid dissolve, carefully breaking up any lumps of solid with the rod. When the solid has dissolved, pour the solution into the flask using a filter funnel. Add water to the flask until the level rises to the graduation mark.'Suggest three further instructions that would improve the overall technique in this account.
Wash the wet rod into the flask after use. Wash the wet beaker into the flask after transfer. Wash the filter funnel, after transfer, into the flask. Use a teat pipette to make up to the mark on the volumetric flask. Ensure the bottom of the liquid meniscus is on the graduation mark. Shake the final solution in the flask.
Suggest one method the student could use for removing soluble impurities from the sample of magnesium hydroxide that has been separated.
Wash with a small amount of cold water
During a titration, a chemist rinsed the inside of the conical flask with deionised water. The water used for rinsing remained in the conical flask.Explain why the water used for rinsing has no effect on the accuracy of the titre.
Water does not affect the titration because it does not change the number of moles of reagents. Water is not a reagent.
Washing with water decreases the concentration of the reagents in the titration flask.Suggest why washing with water does not affect the titre value.
Water does not change the number of moles of either reagent. Water is not a reagent.
A desiccator can be used to dry precipitates as shown in the diagram.Explain briefly how the precipitate in the desiccator becomes dry
Water in the gaseous state from the precipitate absorbed by drying agent.
The melting point of the sample in Question 9.2 was found to be slightly lower than a data-book value.Suggest the most likely impurity to have caused this low value and an improvement to the method so that a more accurate value for the melting point would be obtained.
Water. Press the sample of crystals between filter papers. Give the sample time to dry in air.
Students should understand that the following equation represents a linear relationship. y = mx+c
Where y is the dependent variable, m is the gradient, x is the independent variable and c is the y intercept
In an experiment to determine the acid dissociation constant (Ka) of a weak acid, 25.0 cm3 of an approximately 0.1 mol dm-3 solution of this acid were titrated with a 0.10 mol dm-3 solution of sodium hydroxide. The pH was measured at intervals and recorded.. State the pH value for the anomalous point on your graph. Suggest one reason for this anomaly. Assume that the reading on the pH meter is correct.
pH 8.7 .Ineffective swirling of the mixture
Examples of measurements
ruler. stopwatch. analogue meter. burette
State one reason, other than cost or availability, why water is suitable for washing this solid residue after the filtration.
solid is insoluble in water so it will not dissolve in water.
In assessing uncertainty, there are a number of issues that have to be considered. These include
the resolution of the instrument used. the manufacturer's tolerance on instruments. the judgments that are made by the experimenter. the procedures adopted (e.g repeated readings). the size of increments available (e.g the size of drops from a pipette).
What are readings?
the values found from a single judgement when using a piece of equipment
Examples of readings
thermometer. pH meter. top pan balance. measuring cylinder volumetric flask