Exam 2

Chem 150 NSCC

November 14, 2005

Please circle your answer.

5pts each

1. Hydroquinone is an antioxidant that is also used as a photographic reducer and developer. The normal boiling point of hydroquinone is 310°C. Calculate the pressure at which hydroquinone will boil at 200°C given that its DHvap is 73.38 kJ/mol.

(a) 1.210 x 10-4 mm Hg

(b) 1.35 mm Hg

(c) 22.5 mm Hg

(d) 757 mm Hg

2. The plots below represent vapor pressure versus temperature curves for ethanol, diethyl ether, and water, not necessarily in that order.

a b c

Pressure

(mm Hg)

Temperature (°C)

Based on the relative strengths of the intermolecular forces of attraction of each substance, which is the most likely vapor pressure versus temperature curve for diethyl ether?

(a) curve a

(b) curve b

(c) curve c

(d) can not determine

3. The normal boiling point of pure benzene is found to be 80.10°C. What is the approximate molecular weight of a nonionizing substance if a solution of 3.55g of the substance dissolved in 100g of benzene has a normal boiling point of 80.18°C? Kb = 5.12°C/m for benzene, C6H6.

(a) 20 amu

(b) 500 amu

(c) 2000 amu

(d) 20,000 amu

4. What volume of 3.00 M CH3OH solution is needed to provide 0.320 mol of CH3OH?

(a) 1.04 mL

(b) 9.38 mL

(c) 107 mL

(d) 960 mL

Use the diagram below to answer questions 5-7.

1 atm

P

0.5 atm

50 150 250 350

T (°C)

5. The temperature and pressure at which all three phases can coexist in equilibrium is

(a) 0.25 atm and 110°C

(b) 1.0 atm and 140°C

(c) 1.25 atm and 300°C

(d) 0.50 atm and 130°C

6. The normal boiling point of this substance is approximately

(a) 130°C

(b) 150°C

(c) 300°C

(d) 350°C

7. What phase changes occur when the temperature is held constant at 140°C and the pressure is increased from 0.25 atm to 1.4 atm?

(a) gas ® liquid ® solid

(b) gas ® solid ®liquid

(c) liquid ® solid ®gas

(d) solid ® liquid ® vapor

8. A solution is prepared by dissolving 17.75g sulfuric acid, H2SO4, in enough water to make 100.0mL of solution. If the density of the solution is 1.1094 g/mL, what is the molality?

(a) 0.1775 m H2SO4

(b) 0.1810 m H2SO4

(c) 1.810 m H2SO4

(d) 1.940 m H2SO4

9. Commercial cold packs often contain solid NH4NO3 and a pouch of water. The temperature of the pack drops as the NH4NO3 dissolves in water. Therefore, for the dissolving of NH4NO3 in water,

(a) DHsoln is negative and DSsoln may be negative or positive.

(b) DHsoln is negative and DSsoln is positive.

(c) DHsoln is positive and DSsoln may be negative or positive.

(d) DHsoln is positive and DSsoln is positive.

10. A solution is prepared by dissolving 171g of CdCl2 in enough water to make exactly 250.0mL of solution. If the density of the solution is 1.556 g/mL, what is the weight percent of CdCl2 in the solution?

(a) 7.17%

(b) 44.0%

(c) 56.0%

(d) 68.4%

Problems 11 – 13: Please show ALL your work and place your final answer in the box provided.

11. (15pts) How much energy (kJ) is needed to heat 5.00g of ice from –10.0°C to 30.0°C? The heat of fusion of water is 6.01 kJ/mol, and the molar heat capacity is 36.6 J/molK for ice and 75.3 J/molK for liquid water.

12. (20pts) An aqueous solution containing 100.0g of NaCl and 100.0g CaCl2 has a volume of 1.00L and a density of 1.15 g/mL. The vapor pressure of pure water at 25°C is 23.8 mm Hg, and you can assume complete dissociation for both solutes.

(a) What is the boiling point of the solution?

(b) What is the vapor pressure of the solution at 25°C?

13. (15pts) Sodium has a density of 0.971g/cm3 and crystallizes with a body-centered cubic unit cell. What is the edge length of the cell (picometers)?