Chem Lab Final: pre & post labs
How many moles of NaCl are in 0.58 g of NaCl?
0.00992
Separation of a Mixture: Pre & Post Lab
14 --> 24
You are asked to determine the percentage of the components of a mixture containing sodium iodide, NaI, magnesium hydroxide, Mg(OH)2, and barium sulfate, BaSO4. The total mass of your mixture sample is 4.82g. First you extracted the NaI from the mixture with water and filtered insoluble Mg(OH)2 and BaSO4 from the solution, containing the NaI. After evaporating and drying the filtrate, you find that you have recovered 0.95g of NaI. When you react the insoluble residue of Mg(OH)2 and BaSO4 with 3M HCl, the Mg(OH)2 dissolves. After decanting the supernatant liquid containing aqueous MgCl2 from the insoluble BaSO4 and drying the solid, you recover 2.04g of BaSO4. Finally you add 3M KOH solution to the aqueous solution to precipitate the Mg(OH)2. After filtering thesolution and drying the precipitate, you recovered 1.21g of Mg(OH)2. Calculate the percent NaI in the original sample mixture.
19.7% (.95/4.82)
Laboratory Equipment and Measurements: Pre & Post Labs
2 -->12
5.00 g of an organic compound was found to contain 2.73 g of carbon, 0.46 g of hydrogen and 1.81 g of oxygen. What is its empirical formula?
2.73g C x 1 mol C/12.001g C = .227 mol C .227/.113 = 2 .46g H x 1 mol H/1.008g H = .46 mol H .46/.113 = 4 1.81g O x 1 mol O/15.999g O = .113 mol O .113/.113 = 1 Empirical Formula: C2H4O1
^ Calculate the percent Mg(OH)2 in the original sample mixture.
25.1% (1.21/4.82)
For liquid B, you decide to use a 50 mL burette to once again dispense 5.00 mL of liquid B into a convenient container. You find that the empty container has a mass as 3.05 grams. After obtaining the mass of the empty container you place it beneath the burette and open the stopcock valve. Although you try to stop the flow of liquid B at exactly the 5.00 mL mark you overshoot slightly and end up dispensing 5.14 mL. What is the mass of liquid-B that was dispensed if you continue with the density determination for liquid B and find that the mass of the container plus the 5.14-mL of liquid B is 7.63-grams?
4.58 g (7.63-3.05)
^^ Calculate the percent BaSO4 in the original sample mixture.
42.3% (2.04/4.82)
Molar Mass of a Gas: Pre & Post Lab
44 --> 52
Endo Exo: Pre & Post Lab
54 --> 58
Temperature and Solubility: Pre & Post Lab
60 --> 67
Chemicals in Everyday Life: Pre & Post Lab
69 --> 77
^^^ Calculate the total percent recovery based on the combined masses that were recovered.
87.1% (42.3+25.1+19.7)
^ If its molar mass is about 88 g/mol, what is its molecular formula?
88/[2x12.001 + 4x1.008 + 15.999] = 1.99 = 2 2 x C2H4O1 Molecular Formula: C4H8O2
Calculate the mass, in grams, of 0.4 mol of CuSO4 * 5H2O
99.87
According to the above graph, how many grams of K2SO4 will dissolve in 100 g of water at 30°C?
About 12.5 g of K2SO4
Match the symbols of the following elements: Silver, Gold, Mercury, Rubidium and Iron.
Ag, Au, Hg, Rb, Fe
According to the above graph, how many grams of K2SO4 completely dissolve in 100 g of water at 99°C? Explain your method of estimating
It will completely dissolve at around 24 g. I chose the spot on the dotted line just a little bit below 100°C and then estimated the corresponding value for g on the y axis. ????
What should you do if you break a glass container in the lab?
Sweep up all the broken glass into the dust pan and dispose in the glass disposal box.
2.5 moles of gas,at 1590 mmHg, occupy a volume of 28.00 L. What is the temperature, in ̊C, of the gas? Show your work.
T=(1590x28000)/(2.5x62400)= 285.4K 285.4 - 273.15 = 12.2oC
Which of the following methods would work best for smelling a chemical in lab?
Take a breath of fresh air and then pull the vapors from the top of the flask toward your nose.
If you spill a small amount of chemical on your hands while transferring the chemical from a bottle, you should:
Wash your hand in cool running water for 10-15 minutes
^ a.10g of K2SO4 in 100 g of water at 40°C b.15g of K2SO4 in 100 g of water at 20°C c. 22g of K2SO4 in 200 g of water at 60°C
a) unsaturated b) unsaturated c) unsaturated
Use the information in questions 4 and 5 to calculate the density of the die in terms of grams/cm3.
density = = 1.2 grams/cm3
When is it okay to eat something in the lab room?
never
Which informative symbol might you find on a chemical bottle?
the chemical safety diamond
The analysis of 1.00 g sample of phosphorous oxide, with a molar mass of about 284 g/mol, was found to contain 0.44 g of phosphorous and 0.56 g of oxygen. Calculate the empirical formula and the molecular formula for this phosphorous oxide.
.44g P x 1 mol P/30.974g P = .014 .014/.014 = 1 1 x 2 = 2 .56g O x 1 mol O/15.999g O = .035 .035/.014 = 5/2 (5/2) x 2 = 5 Empirical Formula: P2O5 284/[2x30.974 + 5x15.999] = 2 2 x P2O5 Molecular Formula: P4O10
Chemical Formula: Pre & Post Lab
26 --> 33
Complete and balance the following equations: LiCl(s) + H2SO4(aq) --> AgNO3(aq) + I- (aq) -->
2LiCl(s) + H2SO4(aq) → Li2SO4(aq) + 2HCl(aq) AgNO3(aq) + I-(aq) → AgI(s) + NO3(aq)
Why should you be careful when you heat your NaCl solution to evaporate the water?
If the solution boils too much it could splatter and I would lose some of the sodium chloride.
If you get a large quantity of chemicals spilled down your back, you should:
Immediately go to the safety shower and rinse off for 15 minutes after removing your clothing
It is okay to pipet by mouth so long as the chemical solution is dilute enough to not be hazardous.
f
25.00 mL of a nitric acid solution of unknown concentration is pipetted into a 125-mL Erlenmeyer flask and 2 drops of phenolphthalein are added. A 0.150M sodium hydroxide solution (the titrant) is used to titrate the nitric acid solution (the analyte). If 12.75 mL of the titrant is dispensed from a buret in causing a color change of the phenolphthalein, what is the molar concentration of the nitric acid solution?
0.0765
A 0.397 g sample of potassium hydrogen phthalate, KHC8H404 (molar mass = 204.44 g/mol) is dissolved with 50 mL of deionized water in a 125-mL Erlenmeyer flask. The sample is titrated to the phenolphthalein endpoint with 16.22 mL of a sodium hydroxide solution (see Equation 1 below). HKC8H4O4(aq) + NaOH(aq) --> KNaC8H4O4(aq) + H2O(l) (Equation 1) What is the molar concentration of the NaOH solution?
0.12
A 4.00 g mass of NaOH is dissolved in 5.00 mL of water. A 4.00-mL aliquot of this solution is then diluted to exactly 500 mL of solution using distilled water. What is molar concentration of NaOH in the final diluted solution?
0.16
For liquid B, you decide to use a 50 mL burette to once again dispense 5.00 mL of liquid B into a convenient container. You find that the empty container has a mass as 3.05 grams. After obtaining the mass of the empty container you place it beneath the burette and open the stopcock valve. Although you try to stop the flow of liquid B at exactly the 5.00 mL mark you overshoot slightly and end up dispensing 5.14 mL. What is the density of liquid B in terms of g/mL?
0.891 g/mL (4.58/5.14)
You wish to determine the density of two unknown, immiscible liquids, A and B, at room temperature. To do this you must measure the mass of a known volume of each liquid. For liquid A, you decide to use a volumetric pipet to dispense exactly 5.00 mL into a convenient container. Before dispensing the volume of liquid A into the container, you obtain and record the container's mass as 2.83 grams. After adding the 5.00 mL of liquid A to the container you reweigh the liquid containing container and find the combination to have a mass of 7.71-grams. What is the density of liquid A in g/mL?
0.976 g/mL (4.88/5)
Decide whether each of these reactions is endothermic or exothermic. 1) When two chemicals mix their temperature rises 2) A solid burns brightly and releases heat, light and sound. 3) When two chemicals are mixed their temperature drops. 4) Two chemicals will only react if you heat them continually. 5) Plants take in light energy for photosynthesis. 6) Endothermic or exothermic based on the reaction diagram:
1) exo 2) exo 3) endo 4) endo 5) endo 6) exo
(4) You wish to determine the density of a die (dice). To do this you must measure its mass and volume. To measure its mass you use an available balance. After ensuring that the balance is properly zeroed, you place a weigh boat on the balance and record its mass of 0.55 grams. Next you place the dice in the weigh boat on the balance pan and estimate the total mass of the dice and weigh boat to be 3.01 grams. What is the mass of the dice?
3.01 (total mass of the dice and the weigh boat) - 0.55 (mass of the weigh boat) = 2.46 grams
A saturated solution of NaCl contains 6.60 g salt in 20 mL water at 24 degrees Celsius. Calculate the solubility of NaCl in g solute/100 g H2O (consider water density as 1 g/mL).
33
Titration of Acids and Bases: Pre & Post Lab
35 --> 43
If you did not completely dry the Erlenmeyer flask before weighing the result, how would that affect the final results? Be specific in your answer.
According to the equation M=(mRT)/(PV), the increase in the mass of the condensed gas would also make the molar mass of the unknown sample greater than it actually is.
What chemical test(s) could you use to distinguish solid barium chloride from solid barium sulfate?
Add water or some other solvent: BaCl2 is soluble where-as BaSO4 is not. Add a soluble sulfate salt to each. BaCl2 plus a sulfate salt creates BaSO4 SO42-(aq) + BaCl2(aq) ---> BaSO4(s) + 2Cl-(aq) BaSO4 plus a sulfate creates no reaction. SO42-(aq) + BaSO4(s) ---> BaSO4(s) + SO42-(aq)
Which device should be used as a suction device for pipeting liquids? a. A vacuum pump b. An appropriate sized pipet bulb c. A balloon d. Your mouth
An appropriate sized pipet bulb
According to your data, how is solubility of KNO3 affected by an increase in temperature of the solvent?
As the temperature of the solvent increases so does the solubility of KNO3 at a rate of 2.4086 per °C.
Explain why the temperature at which the first crystals form is recorded.
At that point the solution contains the maximimum amount of solute at that temperature. This gives a solubility value with a corresponding temperature.
The molecular formula of naphthalene is C10H8. What is its empirical formula?
C5H4
A mixture was known to contain three of the four compounds in the table. After the mixture of the three solids was extracted with hot water and filtered, Compound A was obtained by evaporating the filtrate to dryness. When 3M HCl was added to the solid residue, a clear solution resulted. After adding excess 3M NaOH solution to the acid solution, a precipitate was formed. After filtration, the insoluble solid was found to be Compound B. Compound C was recovered by evaporation of the filtrate. None of the compounds (A, B, or C) was soluble in all of the solvents; water, 3M HCl, and 3M NaOH. Identify Compounds A, B, and C. Justify your answer.
Compound A is Benzoic acid because it is the only one soluble in hot water. Both Mg(OH)2 and Zn(OH)2 are soluble in 3M HCl, but Mg(OH)2 is not soluble in 3M NaOH, making it Compound B. Compound C is Zn(OH)2. Na2SO4 is soluble in all of the solvents, and therefore could not have been one of the compounds
List four physical methods that can be used to separate mixtures.
Decantation Filtration Evaporation Extraction
How is a "half-drop" of titrant dispensed from a buret?
If a half-drop is suspended from the buret it is rinsed into the reaction flask
The colors of the chemical safety diamond represent different types of hazards. Match the color to the hazard below: What hazard does the blue diamond represent? What hazard does the red diamond represent? What hazard does the yellow diamond represent? What hazard does the white diamond represent?
Health Hazard Fire Hazard Chemical Reactivity Hazard Any additional hazard information that is needed.
What test could you perform to detect the presence of NH4+ ion? Write a reaction and explain what is expected to happen.
I could test for NH4+ by adding NaOH and finding the scent of NH3 or by using red litmus paper. NH4+(aq) + OH-(aq) ⇔ NH3(aq) + H2O(l)
Briefly describe how you could separate the components of a mixture of benzoic acid and Na2SO4 and recover the two separated substances.
I could use extraction with cold water to remove benzoic acid (non-soluble in cold water) from the Na2SO4 (soluble in cold water). Filtration could then remove the dissolved Na2SO4 with the water by using filter paper, and the benzoic acid would be left behind on top of the paper. Then evaporation (with a hot plate) could be used to remove the water from the Na2SO4. Similarly, left over water can be evaporated from the benzoic acid using a bunsen burner.
After you have performed the above procedures you notice that the die is NOT a perfect cube, as there are depressions on each side and the edges are rounded. Have you over or under estimated the volume of the die? If you use these values, will you overestimate or underestimate the density of the die? Explain
I have overestimated the volume because the depressions and the rounded edges make it so that the lengths of the die in those areas are less than 1.27 cm. As a result I have underestimated the density because the volume (which goes in the denominator when solving for density) is smaller than calculated, therefore the density would be larger than calculated. Also, if a smaller object had the same mass as a larger object, the small object would naturally have a higher density.
A mixture of NaCl, SiO2, and CaCO3 is separated following the procedure given in this experiment. Indicate how each of the following procedural changes would affect the amount of the specified substance recovered. Briefly explain. Hint: Refer to Table 1 for Solubility & Reactivity The recovery of CaCO3 was attempted by adding 3M H2SO4 instead of 3M HCl to the SiO2/CaCO3 residue.
If 3M H2SO4 was added instead of 3M HCl, CaSO4 (not water soluble) will be formed instead of CaCl2 (which is water soluble). As a result much of the CaCO3 will not be filtered out and will instead stay behind with the SiO2 (which would not have reacted to either acid). Less CaCO3 would be recovered.
Do you think that washing soda, Na2CO3, could be used for the same purpose as baking soda, NaHCO3?
It depends, because when either are mixed with HCl the product is almost the same except for an extra NaCl in Na2CO3, therefore both can neutralize HCl. On the otherhand, they are not guaranteed to be similar when mixed with other reactants, such as when being used to cook. Washing soda, Na2CO3, is also not edible.
For liquid B, you decide to use a 50 mL burette to once again dispense 5.00 mL of liquid B into a convenient container. You find that the empty container has a mass as 3.05 grams. After obtaining the mass of the empty container you place it beneath the burette and open the stopcock valve. Although you try to stop the flow of liquid B at exactly the 5.00 mL mark you overshoot slightly and end up dispensing 5.14 mL. Can you still proceed with the density determination for liquid-B or do you have to start over and get exactly 5.00 mL? Explain.
It is possible to continue because you know that the volume is 5.14 and because the mass will be slightly larger because the extra liquid, therefore the answer for density would be the same no matter how much liquid was used.
Which of the two liquids is denser; if combined, which liquid would sink, which would float?
Liquid A is denser, which means it would sink if combined with liquid B. Because liquid B is less dense, it would float on top of liquid A.
Which one of the following figures best represents the expected result when three immiscible liquids are placed in a graduated cylinder together. The density of liquid A is 0.89 g/mL, while liquid B is 1.00 g/mL, and liquid C is 3.89 g/mL.
Liquid C (bottom) --> B --> A
A sample of gas weighs 3.10 g and occupies a volume of 1,272mL at 30 ̊C and 720 mm Hg. What is the molar mass of the gas? Show your work.
M=(3.10gx62400x303)/(1272x720)= 64.0 g/mol
3.0 moles of hydrogen occupy a volume of 67.20L, at standard temperature and pressure (STP, 0 ̊C and 760 mmHg). What is the density of hydrogen at STP? Note: Hydrogen gas is a diatomic molecule. Show your work.
M=dRT/P d=MP/RT d=(2x1.008x760)/(62400x273.15)= 8.99x10-5 g/mL
What do the initials MSDS stand for?
Material Safety Data Sheet
Write the chemical equations for the reaction of Na2CO3 with HCl and the reaction of NaHCO3 with HCl.
Na2CO3 (aq)+ 2HCl (aq)------> 2NaCl (aq) + H2CO3 (s) NaHCO3 (aq)+ HCl (aq) ------> NaCl (aq) + CO2 (g) + H2O (l)
What test could you perform to detect the presence of I- ion?
NaClO is capable of identifying I- ions by freeing I2 from iodide salts. The Iodine will then turn water a reddish brown or mineral water a violet color. 2I-(aq) + ClO-(aq) +H2O(l) → I2(aq) + Cl-(aq) + 2HO-(aq) Iodides will also form a pale precipitate when treated with silver nitrate solution I-(aq) + AgNO3(aq) → Agl(s) + NO3-(aq)
Could you separate the components of a mixture Mg(OH)2 and Zn(OH2) by using 3 M HCL? Briefly explain your answer. post lab question 1 for table
No, because both components are soluble in 3M HCl, and would therefore not be separated in that way. Go a little further in your explanation. Make it clear to someone why them both dissolving is a problem for a separation.
Density of a substance is given by D=m/v. Use this relationship and the ideal gas equation to produce a version of the ideal gas equation that solves for the density of a gas. Show your steps.
PV=nRT D=m/V V=m/D Pxm/D=nRT D = (Pm)/(nRT)
Is the indicator generally added to the titrant or the analyte in a titration?
The analyte
When preparing the buret for titration, the final rinse is with the NaOH titrant rather than with deionized water. Explain the reason for this.
The buret will still have water in it, so the final rinse has to be NaOH in order to get rid of impurities and prevent the solution from being diluted
How would your experiment have been affected if you had used a different initial amount of the unknown compound? Explain. Be specific about how this would affect your results.
The extra molecules and pressure are allowed to escape through a hole. The final result would be unchanged.
The recovery of CaCO3 was attempted by adding 1M KNO3 solution instead of 1M K2CO3 solution to the filtrate, containing aqueous CaCl2.
The insoluble CaCO3 would not form. Instead all the compounds formed would be water soluble and therefore could not be removed using vacuum or gravity filtration. As a result, no CaCO3 would be recovered
What is the purpose of a primary standard in acid/base titration analyses?
The primary standard, or standard acid, is titrated with the base to determine its concentration. You need more explanation of the purpose. Why can't we use something else?
Why is it unwise to haphazardly mix household chemicals or other chemicals?
The resulting chemical reactions could be dangerous explosions or other hazardous reactions.
Define empirical formula.
The simplest whole number ratio of atoms in a chemical formula.
Define the analyte in a titration.
The substance being analyzed (a solution with an unknown concentration)
If all of the vapor had not condensed to a liquid when you cooled the Erlenmeyer flask, how would your calculations have been affected?. Explain. Be specific in how this would affect the result.
The volume will not change in this experiment. The measured mass and therefore the molar mass would be less than the true values.
Why is it necessary to completely dry the Erlenmeyer flask, after removing it from the ice-bath?
The weight of the water will add to the weight of the flask and its contents, therefore the mass of the condensed gas would be higher than it should.
When rinsing a buret after cleaning it with soap and water, should the rinse be dispensed through the buret tip or top opening of the buret?
Through the buret tip
Why should distilled water be used when making chemical tests?
To avoid contaminating the experiment with the extra minerals in undistilled water, which could create inaccurate results.
What chemical test(s) could you use to distinguish sodium chloride (table salt) from sodium iodide (a poison)? Show reactions.
To distinguish sodium chloride and sodium iodide from one another, they could be mixed with silver nitrate, and the color of the resulting reaction would identify the substance. Iodine treated with silver nitrate solution forms a pale yellow precipitate. NaI (aq) + AgNO3 (aq) -----> AgI (s) + NaNO3 (aq) Chloride mixed with silver nitrate forms silver chloride, which is a white compound. NaCl (aq) + AgNO3 (aq) -----> AgCl (s) + NaNO3 (aq) When mixed with sulfuric acid, the chloride salts in sodium chloride will give off a pungent smell and turn moist blue litmus paper red. 2NaCl(s) + H2SO4(aq) ---> 2HCl(g) + 2Na+(aq) + SO42-(aq)
During chemical reactions the bonds between atoms break and new bonds form. Energy must be absorbed to break a bond, so breaking bonds is endothermic. Making new bonds is exothermic because energy is released. If the reaction equation is: 2H(g) --> H2(g) Is this reaction exothermic or endothermic? Use ideas about bonds to explain why.
To make 2H become H2, new bonds must be made and energy is released, so the reaction is exothermic.
When using a chemical in the lab, you should always: a. Keep it at your station so no one else can use it b. Spill them everywhere so everyone will know you did something that day c. Use a small container to transfer chemicals from larger containers d. Return unused chemicals to the original bottle to prevent excess waste
Use a small container to transfer chemicals from larger containers
(5) Since the die is a cube, a regularly shaped geometric object, its volume can easily be determined by performing a direct, physical measurement of its length using a metric ruler. You find the length of one side of the die to be approximately 1.27cm long. Since the die is a cube, its volume can be estimated as being equal to the length of one of its sides cubed (volume = length of a side3). What is the volume of the cube in cm3 using this method?
V = (1.27)3 = 2.0483 = 2.05 cm3
When is it okay to dispose of laboratory materials down the drain of the lab sinks?
When the instructor informs you that it is okay at the beginning of lab.
According to the above graph, will 10g of K2SO4 completely dissolve in 100g of water at 50°C? Explain
Yes, 10g of K2SO4 will completely dissolve in 100 g of water at around 20°C, so it will already be dissolved when it is 50°C. ????
According to the following graph, tell if each of these solutions would be saturated or unsaturated:
a linear dot graph w/ C on bottom (x) & solubility on side. (y) (10,8) (20, 11) (40, 14) (60, 17) (80, 21) (90, 23)
What things should you do before leaving the lab? a. wipe down your lab bench b. throw out all the glassware that you used c. wash your hands d. place wet glassware on the drying racks e. open all the chemical bottles f. dispose of all waste and recovered solutions
acdf
For endothermic reactions: Reactants + __________ --> products
energy
For exothermic reactions: Reactants + __________ --> products
energy
It is okay to wear flip-flops in the lab room
f
It is okay to start an experiment before the instructor ever gets to lab.
false
The chemistry stock room will not rent or loan goggles.
false
What should you do when a very small container of alcohol catches fire in the lab?
place a cover on the container
Which of the following items is not appropriate to wear to lab? a. Long sleeved shirt b. Tank tops c. Full length pants d. Safety goggles
tank tops
