Chem 121 Fritsch

Ch 12 and 14 Practice Problems - KEY The following problems are intended to provide you with additional practice in preparing for the exam. Questions come from the textbook, previous quizzes, previous exams, and other sources. A solutions manual is supplied in a separate document. 1. Which of the following has the substances in order of decreasing boiling point? CH3CH2CH2CH3 CH3CH2CH2OH HOCH2CH2OH CH3CH2CH2Cl

Decreasing boiling point Answer: HOCH2CH2OH CH3CH2CH2OH CH3CH2CH2Cl CH3CH2CH2CH3 2. What property of water allows a needle to float on it without sinking? Answer: surface tension

3. The scent wintergreen includes methyl salicylate (C8H8O3) and has a vapor pressure of 2.52 mm Hg at 70 °C. The vapor pressure at 100 °C is 12.8 mm Hg. Calculate the vapor pressure of methyl salicylate at 154 °C.

Chem 121 Fritsch

4. A 67.83 g sample of CO2 was compressed into a 2.61 L flask at 10.0 °C. Which answer correctly reflects the contents of the flask? You may find the phase diagram at the right helpful. (The density of CO2(l) at 10 °C and 10 atm is 0.77 g/mL.) A. Solid only B. Liquid only C. Gas only D. A mixture of solid and liquid E. A mixture of solid and gas F. A mixture of liquid and gas Answer:_____F____

5. Answer the following questions, regarding the following two series of compounds.

I. CH3OH Xe CCl4 PF3 II. Cl2 H3PO4 CH3Br CH2F2

a. For each series, place in order of increasing inter-molecular forces. b. For each series, which will have the highest vapor pressure at 25 °C?

Chem 121 Fritsch

6. Fractional distillation is used to separate the various hydrocarbons in crude oil. You set up a fractional distillation where 132.78 g of ortho-xylene was combined with 87.69 g of para-xylene yielding a homogeneous mixture in your distillation apparatus. You heat the liquid to 120 °C. What is the mole fraction of para-xylene in the vapor as it enters the fractionating column?

o-xylene p-xylene

molecular formula C8H10 C8H10

density (g/mL) 0.879 0.861

Normal b.p. (°C) 145 138

Pvap(120 °C), mmHg 391.5 471.0

H°vap (kJ/mol) 36.24 35.70

Chem 121 Fritsch

7. For the following compounds, place an X in the box for all of the intermolecular forces present for that compound.

Hydrogen bonding Dipole-dipole London Forces

1-octanol

X X X

H3C-CCl3 X X

SO2 X X

N2 X

oxalic acid

X X X

b. Which will have the highest boiling point? ______oxalic acid__________________

8. Calculate the enthalpy of vaporization (ΔHvap in kJ/mol) of diethyl ether which has a normal boiling point of 34.6 °C and 0.703 atm at 25.0 °C.

Chem 121 Fritsch

9. For the following combinations of substances, determine if a homogenous mixture would be observed. Will the solid be soluble in the solvent? Will the two liquids be miscible or immiscible (not miscible)? Justify your answer inside each box to the right.

Cmpd 1 Cmpd 2 Homogeneous (Y/N)

Justification

a. Br2(s) CHCl3(l) Y Strong dispersion forces in both

b. C5H12 C2Cl4 Y Strong dispersion forces in both

c. acetone H2O Y Both molecules have dipole moments

d. C5H12 H2O N IMF mis-match – dispersion to H-bonding/dipole

e. 0.05g NaCl

100 mL CH3OH

Y Likely soluble based on quantities and ion dipole interactions

f. 0.50g NaCl

10 mL C5H12

N Not likely based on quantities and IMF mis-match

10. The freezing point of an aqueous solution prepared with 176 g H2O was determined to be –5.02 °C after 20.18 g of a soluble, unknown ionic solid was added. Which ionic compound was added: sodium nitrate (85 g/mol) or sodium phosphate (196 g/mol)? Support your choice with a calculation.

Chem 121 Fritsch

11. Calculate the total heat (in Joules) needed to

convert 35.63 g of H2O(s) at -14.6 °C to H2O(l) at

47.2 °C.

12. Place the following in order of increasing kJ/mol.

a) ΔHvap(H2O) +40.65

b) ΔHvap(F2) +6.6

c) ΔHvap(GeH4) +14.06

d) ΔHfus(CH2Cl2) +28.6

H2O(s) H2O(l)

Csp (J/g•°C) 2.01 4.184

density (g/mL) 0.917 1.000

Chem 121 Fritsch

13. Calculate the vapor pressure of ortho-xylene and para-xylene, C8H10, at 87 °C. Report your answer in mm Hg.

o-xylene p-xylene

molecular formula C8H10 C8H10

density (g/mL) 0.879 0.861

Normal b.p. (°C) 145 138

Pvap(120 °C), mmHg 391.5 471.0

H°vap (kJ/mol) 36.24 35.70

Chem 121 Fritsch

14. A 154 cm3 ice cube at -25.0 °C is added to a thermally isolated container with 400.0 mL H2O(l) at 30.0 °C. What mass (in grams) of ice remains in the container

after thermal equilibrium is reached?H°fus =6.01 kJ/mol)

15. Place the following solution in order of increasing freezing point. 0.100 m sucrose 0.100 m AgNO3 0.200 m CuSO4 0.100 m MgCl2

H2O(s) H2O(l)

Csp (J/g•°C) 2.01 4.184

density (g/mL) 0.917 1.000

Chem 121 Fritsch

16. Consider a three component mixture containing 27.83 mL water, 32.35 mL ethanol and 42.64 g sucrose (C12H22O11, 342.30 g/mol). Calculate the mole fraction of ethanol in the vapor above this solution at 50 °C.

H2O EtOH

molar mass (g/mol) 18.02 46.07

density (g/mL) 1.000 0.789

Normal b.p. (°C) 100 78.4

Pvap (50 °C), mmHg 92.5 208.2

H°vap (kJ/mol) 40.67 38.56

Chem 121 Fritsch

17. An experiment was conducted with a sealed container of ethanol and varying temperature. Ethanol (C2H6O, 1.41 g) was added to a 5.6 L bottle that was sealed, evacuated to remove any air, and placed in an ice bath containing H2O(l) and H2O(s). Eventually the vapor pressure inside the flask stabilized at 0 °C. (normal b.p. = 78.4 °C, normal f.p. = -114 °C, ΔHvap = +38.56 kJ/mol) a. What mass of ethanol was in the vapor phase at 0 °C? b. What mass of ethanol was in the liquid phase at 0 °C? c. The bottle was placed in a beaker of boiling water. What was the vapor pressure inside the bottle in the beaker of boiling water?

Chem 121 Fritsch

18. For the following two molecules, one is soluble in water and the other is soluble in hexane. Which is soluble in water and which is soluble in hexane. Explain the difference in solubility between the two compounds. a.

Nonanoic acid

Hexane soluble, has significant London forces as its IMFs

b.

glucose Water soluble, lots of hydrogen bonding as its primary IMF

19. The solubility of caffeine in water varies with temperature. At room temperature its solubility is 2 g/ 100 mL while at 100 °C the solubility is 66 g/ 100 mL. Explain this difference. A cup of coffee can contain 95 mg of caffeine per 8 oz. Is the solution saturated, unsaturated or super saturated in caffeine?

Chem 121 Fritsch

20. An aqueous solution was prepared by 17.83 g lithium carbonate dissolving in 84.71 g H2O. Answer the following question regarding this solution. a. Calculate the molality b. Calculate the mole fraction of H2O. c. Calculate the percent mass of lithium carbonate. d. Calculate the vapor pressure above the solution at 45 °C. (P°H2O = 71.9 mmHg at 45 °C) e. Calculate the boiling point of the solution.

Chem 121 Fritsch

21. Calculate the total pressure at 50 °C above a mixture of methanol (CH3OH) and acetic acid (C2H6O2) where the masses used of each compound are the same but of sufficient quantity that both compounds are present as both liquids and vapors. (P°CH3OH = 400 mmHg at 50 °C and P°C2H4O2 = 40.60 mmHg at 50 °C).

22. Calculate the freezing point and boiling point of a 0.57 m solution of sodium chloride that was dissolved into ethanol. (mp = -114 °C, bp = 78 °C, Kf = 1.99 °C/m ,Kb = 1.22 °C/m)

Chem 121 Fritsch