Chem I Lab Final
How will you determine the mass of the vaporized liquid?
After vaporizing the liquid. the flask is allowed to cool and the the apparatus contains the vapor is weighed. The known mass of the empty apparatus is subtracted from this and this is the mass of the vapor.
When rinsing a buret after cleaning it with soap and water, should the rinse be dispensed through the buret tip or the top opening of the buret? Explain.
By rinsing through the tip you ensure no waste collects at the tip while water drains out the buret
Part D.1. All of the CuO does not react with the sulfuric acid. Will the reported percent recovery of copper in the experiment be too high or too low? Explain.
If not all the copper oxide reacts with the sulfuric acid this will result in the copper recovery percentage being too low. This is because some of the copper remains as copper oxide and will not react with the magnesium that will be added and as a result not return to its original copper state. This failure to recover everything possible will lead to the percent recovery being lower than it should be
The aluminum foil is pierced several times with large pencil-size holes instead of pin-size. a. How will this oversight in the procedure affect the mass of vapor measured in Part B.3, too low, too high, or unaffected? Explain. b. Will the reported molar mass of the liquid be reported too low, too high, or unaffected? Explain.
A large amount of vapor will escape so the reported mass will be too low and the molar mass will be too low as well
Because of the small particle size of the CaC2O4*H2O precipitate, some of it pass through the filter and is not collected. How does this loss of precipitate affect the reported percentage of the limiting reactant in the salt mixture?
A loss of precipitate would result in the reported percentage of the limiting reactant in the salt mixture being smaller than the actual percentage of the limiting reactant
What is the procedure of heating metal to an exact but measured temperature?
A test tube should have 10 to 30 g of metal transferred into it. After, the test tube is transferred to a beaker that filled with water on a hot plate. The water should be allowed to boil for 10 minutes so the metal may reach equilibrium
Why must the reaction of copper reacting with nitric acid be performed in a fume hood.
NO2 is produced and this is a toxic gas that should no be inhaled so the experiment is conducted under a fume hood so the gas does not enter the rest of the lab
The sample in Part B was not centrifuged. Why? Perhaps the student chemist had to be across campus for another appointment. Because of the student's "other priorities" the percent recovery of copper in the experiment will decrease. Explain why.
The reaction was not allowed to complete. This results in not all of the copper hydroxide forming and leads to less copper being recovered at the end of the experiment because some of it still remains as copped nitrate and will not form copper oxide in the next reaction
The unknown liquid is volatile. If some of the liquid evaporates between the time the liquid is delivered to the beaker and the time its mass is measured. What will happen to the reported density?
The reported density will be too low, assuming the volume has already been recorder. The mass be divided by a volume that is larger than the actual amount that is left in the beaker. This will result in a lower density being recorded.
The drying oven, although thought (and assumed) to be set at 125°C, had an inside temperature of 84°C. How will this error affect the reported percent by mass of the limiting reactant in the salt mixture . . . too high, too low, or unaffected? Explain.
The reported mass be too high. The CaC2O4•H2O will not be completely dry in the end so the mass will be too high, This causes the limiting reactant to be too high and the percent limiting reactant will be reported as too high.
What is the size and volume of a small, clean test tube?
The size of a small testate is 75 mm. The volume of the small test tube is 30 mL.
How is the limiting reactant determine in the experiment?
The supernatant created after using the filtering vacuum is separated into 2 vials -CaCl2 is added to 1 vial and if a precipitate forms then CaCl2 is the limiting reactant -K2C2O4 is added to the other vial and if a precipitate forms the K2C2O4 is the limiting reactant
Mass measurements of the crucible, lid, and sample are performed only at room temperature. Why is this technique necessary for a gravimetric analysis?
The weights are calibrated for room temperature, as well the initial wight for the sample is taken at room temperature so the following measurement should be taken at a similar status
Identify at least 5 observations that are indicative of a chemical reaction.
-Gas evolution -change in color -change in odor -appearance of a precipitate or disappearance -heat given off or absorbed
Sulfuric acid has a dual role in the chemistry of this experiment. What are its two roles in the recovery of the copper metal?
-Results in copper oxide having its oxygen removed to form copper sulfate and water. -Sulfuric acid works to speed up this reaction so the process of copper recovery is quicker
Three colorless solutions in test tubes, with no labels, are in a test tube rack on the laboratory bench. Lying beside the tests tubes are three labels: 0.10 M Na2CO3, 0.10 M HCl, and 0.10 M KOH. You are to place the labels on the test tubes using only the three solutions present. Here are your tests: • A few drops of the solution from test tube 1 added to a similar volume of the solution in test tube 2 produces no vis- ible reaction but the solution becomes warm. • A few drops of the solution from test tube 1 added to a similar volume of the solution in test tube 3 produces carbon dioxide gas. Identify the labels for test tubes 1, 2, and 3.
1- 0.1M HCl 2- .1M KOH 3- .1M Na2CO3
Three student chemists measured 50.0 mL of 1.00 M NaOH in separate Styrofoam coffee cup calorimeters (Part B). Brett added 50.0 mL of 1.10 M HCl to his solution of NaOH; Dale added 45.5 mL of 1.10 M HCl (equal moles) to his NaOH solution. Lyndsay added 50.0 mL of 1.00 M HCl to her NaOH solution. Each student recorded the temperature change and calculated the enthalpy of neutralization. Identify the student who observes a temperature change that will be different from that observed by the other two chemists. Explain why and how (higher or lower) the temperature will be different.
Dale will see a higher temperature change because the NaOH is the limiting reactant so only a certain amount will react before the process stops. So the difference in the heat would depend on the amount put into the cup. Dale will have the same reaction occur in his cup, but because he has a smaller amount of solution going through the same energy changes as the other 2 cups his heat will be higher because it is composed into a smaller amount.
Calcium chloride, a deliquescence salt, is used as a desiccant in laboratory desiccators to maintain a dry environment. Explain.
Deliquescent salts readily absorb water so calcium chloride will absorb any moisture in the environment and make sure that the environment stays dry
The crucible and lid are handled exclusively with crucible tongs in the experiment. How does this technique maintain the integrity of the analysis?
Ensures no extra oils are being added on to the walls of the crucible because this can cause the measured mass to be too high
How will you determine the volume of the capsized liquid?
Fill the flask to the brim with water and transfer this amount to a graduated cylinder and record the volume
What is the purpose of firing the crucible?
Firing the crucible before weighing removes any oils or water that may be present on its wall and make sure they do not add weight on to the crucible when taking its mass
If the step for digesting the precipitate were omitted, will the reported "percent limiting reactant" in the salt mixture be too high, too low, or unaffected? Explain.
Not digesting the precipitate will cause more precipitate to be lost during the filtering process and the amount of CaC2O4 to be calculated as higher than it actually is. This would lead to the amount of limiting reactant to calculated as a lower number as well as the percent of limiting reactant
Explain why it is quantitatively not acceptable to titrate each of the vinegar samples with the NaOH titrant to the same dark pink endpoint.
Once the solution changes to pink and remains pink after the 30 seconds, the solution endpoint has been reached. If titrated past this point then the solution has become over saturated and will interfere with calculations
Describe the technique for testing the odor of a chemical?
Place your hand above the test tubes opening and use a gentle wafting motion to wave the scent to your nose
What test reagent used in the substance identification experiment will distinguish a soluble Cl- salt from a soluble SO4^2- salt? What is the distinguishing observation?
Silver nitrate can be used to distinguish between the soluble CL- salt and a soluble SO4^2- salt. The sulfate ion will form a clear precipitate and with chloride ion will form a white precipitate.
A chemist often uses a white card with a black mark to aid in reading the meniscus of a clear liquid. How does this technique make the reading more accurate? Explain.
The black background reflects off the bottom of the meniscus and helps define the liquid
Previously boiled, deionized water is unavailable. In a hurry to pursue the analysis, deionized water (not boiled) is added. How does this attempt to expedite the analysis affect the reported percent acetic acid in vinegar: too high, too low, or unaffected? Explain.
The bubbles will read as part of the volume of the base. This will lead to the percent acetic acid being too high because less base will have been dispensed to reach equilibrium than the actual number so the number of moles of acetic acid will be recorded to be too high.
The 20 mL of water added to the Erlenmeyer flask is to be previously boiled, deionized water. Since water does absorb CO2 from the atmosphere and since CO2 dissolved in water causes it to be slightly acidic, will the use of deionized water that has not been previously boiled cause the mass of acetic acid in the vinegar to be calculated as too high or too low? Explain.
The calculated mass of the acetic acid in the vinegar will be too high. The carbon dioxide and oxygen will cause it to be too acidic, leading to more NaOH to be dispensed and the believed mass of acetic acid to be too high
The endpoint of the titration is overshot! Does this technique error result in an increase, a decrease, or have no effect on the reported percent acetic acid in the vinegar? Explain.
This imbalance would result in the reported percent acetic acid in vinegar being too high
Suppose that after delivery several drops of the water cling to the inner wall of the pipet. Will this technique error increases, decrease or have no effect on the reported density of water?
This will result in a lower density being reported. Not all of the water will be dispensed so a low mass of water will be reported. and divided by a too large volume resulting a low density.
Digesting a precipitate
Warm the precipitate for 20 minutes to form cleaner and larger particles to make filtering more efficient
Stoichiometric Point
When a reaction is complete because the stoichiometric amount of reactants have combined
Endpoint
When the indicator changes color due to change in pH
In preparing the buret for titration, the final rinse is with the NaOH titrant rather than with deionized water. Explain.
You rinse with NaOH during the final rinse because whenever you clean, a small amount of water remains in the buret and must be cleaned out before the experiment begins, or it may alter the values
The color change at the endpoint should persist for 30 seconds. Explain why the time lapse is a good titration technique.
Could be because the pH changes. The color may change initially because the acid is reacting with a small amount of the solution but as time goes on this goes way as the acid mixes with the rest of the solution. The color must persist to ensure all of the solution is reacting.
When the NaOH solution is added, Cu(OH)2 does not precipitate immediately. What else present in the reaction mixture from Part A reacts with the NaOH before the copper (II) ion?
H2O. Cu(No3)2 is aqueous and has Cu(2+) and NO3(-) ions. When NaOH is added, Na+ ions react with the No3- ions to form the NaNo3, and the Cu(2+) ions react with the OH- ions to form Cu(Oh)2 which is light blue precipitate. It is possible that these H+ and OH- ions that form water started to react with NaOH before Cu(No3)2 did. But the reaction, other than that, needed time
Jacob couldn't find the 6 M H2SO4, so instead substituted the 6 M HNO3 that was available. What change was most likely observed as a result of this decision.
HNO3 cannot be use in the redox titration because it oxidizes itself. Unlike HCl, it isn't volatile, and doesn't take part in the reaction
Because of the porosity of the filter paper some of the CaC2O4•H2O precipitate passes through the filter paper. Will the reported percent of the limiting reactant in the original salt mixture be reported too high or too low? Explain.
If some the CaC2O4•H2O is lost then the calculated number of moles will be lower so the mass of the limiting reactant will be lower and the percent limiting reactant will be too low
In measuring the specific heat of a metal, Josh used the highest measured temperature for calculating the metal's specific heat rather then the extrapolated temperature. Will this decision result in a higher or lower specific heat value for the metal?
If used highest temp instead of extrapolated temp the (change in T) would be smaller for both. If divide by smaller value to get specific heat, the value would be larger than expected.
When a metal at a higher temperature is transferred to water at a lower temperature, heat is inevitably lost to the calorimeter (Figure 25.4). Will this unmeasured heat loss increase or decrease the calculated value of the specific heat of the metal? Explain. See equation 25.5.
The calculated specific heat will be too low. The water temp increase is used to determine the heat lost by the metal. Due to the heat gained by the water being lower than it actually is so the heat lost by the metal will be too low in turn making the specific heat too low.
If the vinegar were measured volumetrically (e.g., a pipet), what additional piece of data would be needed to complete the calculations for the experiment?
The density of the vinegar would need to be converted from mL to the amount of grams used
The solid is not completely submerged in the water. How will this effect the reported density of the solid?
The density will be larger than the actual one. This is because the mass will be divided by a smaller volume resulting a larger density.
The enthalpy of neutralization for all strong acid-strong base reactions should be the same within experimental error. Explain. Will that also be the case for all weak acid-strong base reactions? Explain.
The enthalpy is the same for all strong acid-strong base reactions because they solutions completely disassociate. This will not be the same for weak acid-strong base reactions because the acid does not completely dissociate.
Explain why the extrapolated temperature is used to determine the maximum temperature of the mixture rather than the highest recorded temperature in the experiment. See Figure 25.5.
The extrapolated temperature is used because the heat is likely to escape so the loss needs to be taken into account. So the heat that went into increasing the temperature of the substance is accurately calculated.
How will you determine the temperature of the vaporized liquid in this experiment
The flask is put in a beaker water and brought to a gentlefolks boil. Once vapor is no longer seen escaping through the holes in the foil the water is allowed to boil 15 more minutes and then the temperature of the water is recorded
The 200-mm test tube also contained some water (besides the metal) that was subsequently added to the calorimeter (in Part A.4). Considering a higher specific heat for water, will the temperature change in the calorimeter be higher, lower, or unaffected by this technique error? Explain.
The mass of the vapor will be measured as too low and in turn it's molar mass will be reported as too low
What is the criterion for clean glassware?
The glassware should have no residual chemicals left within or on it
In determining the percent acetic acid in vinegar, the mass of each vinegar sample is measured rather than the volume. Explain.
The mass of vinegar is needed to measure the percentage composition because the mass of acetic acid and vinegar is needed not volume
The buret is filled with the NaOH titrant and the initial volume reading is immediately recorded without waiting the recommended 10-15 seconds. However in Part B.1, the 10-15 second time lapse does occur before the reading is made. Does this technique error result in an increase, a decrease, or have no effect on the reported percent acetic acid in the vinegar? Explain.
The initial reading will be too high. This will result in a larger amount of NaOH to be believed to have been dispensed than there actually was. This will cause the reported percent of acetic acid in vinegar to be too high
The flask is completely filled with vapor only when it is removed from the hot water bath in Part B.3. However, when the flask cools, some of the vapor condenses in the flask. As a result of this observation, will the reported molar mass of the liquid be too high, too low, or unaffected? Explain.
The molar mass will be unaffected because the vapor will remain inside the flask just in a different state. The weight will not be negatively affected by this and the molar mass will be unaffected as well.
The pressure reading from the barometer is recorded higher than it actually is. How does this affect the reported molar mass of the liquid: too high, too low, or unaffected? Explain.
The number of moles will be calculated as too high and the molar mass will be calculated as a too low number because the mass will be divided by a larger number.
Suppose the thermometer is miscalibrated to read 0.3°C higher than actual. Does this error in calibration result in the molar mass of the vapor in the flask being reported as too high, too low, or as unaffected? Explain.
The number of moles will be calculated as too low because the temperature will be too high. In turn the molar mass will be too high because the mass will be divided by a smaller number than it should be
If the volume of the flask is assumed to be 125 mL instead of the measured volume, would the calculated molar mass of the unknown liquid be too high, too low, or unaffected by this experimental error? Explain.
The number of moles will be too low due to the low assumed volume and the molar mass will be too high because of the low number of moles.
From the time the mass of the flask is first measured in Part A.1 until the time it is finally measured in Part B.3, it is handled a number of times with oily fingers. Does this lack of proper technique result in the molar mass of the vapor in the flask being reported as too high or too low or as unaffected? Explain.
The oil will add weight on to the flask and this will be interpreted as a mass being added by the vapor. The mass of the vapor will then me reported as too high and the molar mass as well.
A drop of NaOH titrant, dispensed from the buret, adheres to the wall of the Erlenmeyer flask but is not washed into the vinegar with the wash bottle. Does this error in technique result in the reported percent of acetic acid being too high, too low, or unaffected? Explain.
The percent of acetic acid will be reported as too high. More NaOH will have been believed to have been added than in actuality so it will be believed that there is a larger amount of acetic acid than there actually is.
A couple of drops of water were accidentally placed on the properly folded filter paper before its mass was measured. However, in Part A.6, the CaC2O4•H2O precipitate and the filter paper were dry. As a result of this sloppy technique, will the mass of the limiting reactant be reported too high, too low, or remain unaffected? Explain.
The reported mass of limiting reactant will be too low. This is because the droplets of water will add weight to the reorder weight of the filter paper and a large number will be subtracted from the combined weight of the precipitate and filter paper
The CaC2O4•H2O precipitate is not completely air-dried when its mass is determined. Will the reported mass of the limiting reactant in the original salt mixture be reported too high or too low? Explain.
The reported mass of the limiting reactant will be too high. A larger recorded mass of CaC2O4•H2O results in a larger calculated mass of the limiting reactant in the original salt mixture.
Excessive quantities of wash water are added to the CaC2O4•H2O precipitate. Will the mass of the CaC2O4•H2O be reported too high, too low, or remain unaffected? Explain.
The reported mass will be too low. Some CaC2O4•H2O dissolves each time water is added. Adding excessive amount of water results in a larger loss of CaC2O4•H2O.
Part A.1. The 200-mm test tube also contained some water (besides the metal) that was subsequently added to the calorimeter (in Part A.4). Considering a higher specific heat for water, will the temperature change in the calorimeter be higher, lower, or unaffected by this technique error? Explain.
The temperature change in the calorimeter will be unaffected by this technique error. The original specific heat of water is 4.18 J/g•°C, but if this value increases that means that water will take more energy to change the temperature. Therefore, an increase in the amount of water does not make a difference because the specific heat of water was also increased.
A20-mL volume of previously boiled, deionized water is added to the Erlenmeyer flask to prepare the sample for titration. Explain why this volume is not critical to the analysis.
The water does not affect the number of hydronium and hydroxide ions needed to reach equilibrium. The water is only needed to assist in judging when the endpoint has been reached.
A chemist should wait 10-15 seconds after dispensing a volume of titrant before a reading is made. Explain why the wait is good laboratory technique.
There may be fluid sticking to the sides. Some time should be given for the fluid to fall down before reading so the measurement is accurate.
The wall of the flask is periodically rinsed with the previously boiled, deionized water from the wash bottle. Does this titrimetric technique result in an increase, a decrease, or have no effect on the reported percent acetic acid in the vinegar? Explain.
There will be no effect on the reported percent of acetic acid. The water does not affect the balance of the hydronium and hydroxide ions
A reagent bottle on the shelf labeled 0.5 M NaCl was used in place of the 0.5 M CaCl . Assuming C2O4^2- to be in excess, what would be observed as a result of using this wrong reagent in this test? Explain.
There will be no visible reaction or precipitate formation. The Na+ will not react with the C2O4^2- to form a precipitate.
In a hurry to complete the experiment, Anna withdrew two volumes of solution from Part A.2 before the precipitate had settled. As a result, what dilemma might she have encountered in Part B? Explain.
There will still be Ca^2+ and C2O4^2- in the supernatant so a will precipitate will form in both vials when testing for the limiting reactant and she will not be able to determine what the limiting reactant is
Primary Standard
a highly purified solid compound used to check the concentration of the known solution in a titration
Three colorless solutions in test tubes, with no labels, are in a test tube rack on the laboratory bench. Lying beside the test tubes are three labels: silver nitrate, AgNO3; hydrochloric acid, HCl; and sodium carbonate, Na2CO3. You are to place the labels on the test tubes using only the three solutions present. Here is your analysis procedure: • A portion of test tube 1 added to a portion of test tube 2 produces carbon dioxide gas, CO2. • A portion of test tube 2 added to a portion of test tube 3 produces a white silver carbonate precipitate. a. On the basis of your observations how would you label the three test tubes? b. What would you expect to happen if a portion of test tube 1 is added to a portion of test tube 3?
a. 1- HCl 2- Na2CO3 3- AgNO3 b.I would expect AgCl, a white precipitate, to form.
Predict what would be observed and why from from an aqueous picture for each of the following (all substances are water soluble) a. potassium carbonate and hydrochloric acid b. zinc chloride and silver nitrate c. magnesium chloride and sodium hydroxide d. ammonium nitrate and sodium hydroxide
a. water would form and gas would be emitted b. a precipitate of insoluble silver chloride forms c. insoluble solid magnesium hydroxide forms d. ammonia gas forms. This gas possesses an odor
Secondary Standard
not as pure as primary, concentration is adjusted to an exact concentration against a primary standard