Q: A solution is made by dissolving 200 g of glucose (MW = 180.2 g/mol) in 500 mL of water at 25 °C. The vapor pressure of pure water at 25 °C is 23.76 mmHg and the density of water is 1.0 g/mL. What is the vapor pressure of the solution? (Assume glucose is a nonvolatile solute.)

$$P_{solution} = 23.76 \ \text{mmHg} \times \left(1 - \frac{200/180.2}{(500/18.02)+(200/180.2)}\right) = 22.2 \ \text{mmHg}$$

Q: What is the vapor pressure of a solution containing 30 g of methanol (MW = 32.04 g/mol) and 40 g of ethanol (MW = 46.07 g/mol)? The vapor pressure of pure ethanol is 44 mmHg and pure methanol is 94 mmHg.

$$P_{solution} = (0.484 \times 94) + (0.516 \times 44) = 67.2 \ \text{mmHg}$$

Q: If you add 100 g of NaCl to 2 L of water to cook pasta, what temperature will the saltwater boil at? ($$K_b of $$water = 0.52 °C/m)

$$\Delta T_b = i K_b m = 2 \times 0.52 \times \frac{100/58.44}{2} = 0.89 \ ^\circ\text{C}$$; $$T_{boil} = 100 + 0.89 = 100.89 \ ^\circ\text{C}$$

Q: Candy makers use the boiling point of sugar solutions to determine the concentration of sugar in the solution. If taffy is cooked until it reaches a boiling point of 132 °C, what is the molality of sugar in the solution? ($$K_b of $$water = 0.52 °C/m)

$$\Delta T_b = 132 - 100 = 32 \ ^\circ\text{C}$$; $$m = \frac{32}{0.52} = 61.5 \ \text{mol/kg}$$

Q: How many grams of sodium chloride would you have to add to 500 mL of water to lower the freezing point to -5 °C? ($$K_f of $$water = 1.86 °C/m)

$$\Delta T_f = i K_f m = 2 \times 1.86 \times m = 5$$; $$m = \frac{5}{3.72} = 1.34$$; $$\text{mol NaCl} = 1.34 \times 0.5 = 0.67$$; $$\text{g NaCl} = 0.67 \times 58.44 = 39.2 g$$

Question 1

Which of the following best describes a colligative property?

Accepted Answer

A property that depends only on the number of solute particles in solution.

Question 2

A solution is made by dissolving 200 g of glucose (MW = 180.2 g/mol) in 500 mL of water at 25 °C. The vapor pressure of pure water at 25 °C is 23.76 mmHg and the density of water is 1.0 g/mL. What is the vapor pressure of the solution? (Assume glucose is a nonvolatile solute.)

Accepted Answer

$$P_{solution} = 23.76 \ 	ext{mmHg} 	imes \left(1 - \frac{200/180.2}{(500/18.02)+(200/180.2)}ight) = 22.2 \ 	ext{mmHg}$$

Question 3

Which equation correctly expresses Raoult’s Law for the vapor pressure of a solution containing a nonvolatile solute?

Accepted Answer

$$P_{solution} = X_{solvent} P^0$_{solvent}$$

Question 4

A solution is made by dissolving 50.0 g CaCl$$_2 in 500 g of $$water at 37°C. The vapor pressure of pure water is 47.1 torr at 37°C. What is the main reason the vapor pressure of the solution is lower than that of pure water?

Accepted Answer

The CaCl$$_2$$ dissociates into more particles, increasing the number of solute particles and lowering the vapor pressure.

Question 5

What is the vapor pressure of a solution containing 30 g of methanol (MW = 32.04 g/mol) and 40 g of ethanol (MW = 46.07 g/mol)? The vapor pressure of pure ethanol is 44 mmHg and pure methanol is 94 mmHg.

Accepted Answer

$$P_{solution} = (0.484 	imes 94) + (0.516 	imes 44) = 67.2 \ 	ext{mmHg}$$

Question 6

If you add 100 g of NaCl to 2 L of water to cook pasta, what temperature will the saltwater boil at? ($$K_b of $$water = 0.52 °C/m)

Accepted Answer

$$\Delta T_b = i K_b m = 2 	imes 0.52 	imes \frac{100/58.44}{2} = 0.89 \ ^\circ	ext{C}$$; $$T_{boil} = 100 + 0.89 = 100.89 \ ^\circ	ext{C}$$

Question 7

Candy makers use the boiling point of sugar solutions to determine the concentration of sugar in the solution. If taffy is cooked until it reaches a boiling point of 132 °C, what is the molality of sugar in the solution? ($$K_b of $$water = 0.52 °C/m)

Accepted Answer

$$\Delta T_b = 132 - 100 = 32 \ ^\circ	ext{C}$$; $$m = \frac{32}{0.52} = 61.5 \ 	ext{mol/kg}$$

Question 8

How many grams of sodium chloride would you have to add to 500 mL of water to lower the freezing point to -5 °C? ($$K_f of $$water = 1.86 °C/m)

Accepted Answer

$$\Delta T_f = i K_f m = 2 	imes 1.86 	imes m = 5$$; $$m = \frac{5}{3.72} = 1.34$$; $$	ext{mol NaCl} = 1.34 	imes 0.5 = 0.67$$; $$	ext{g NaCl} = 0.67 	imes 58.44 = 39.2 g$$

Question 9

Which of the following best describes osmotic pressure?

Accepted Answer

The pressure required to stop the flow of solvent through a semipermeable membrane.

Question 10

A red blood cell is placed in a hypotonic solution. What will happen to the cell?

Accepted Answer

Water will move into the cell, causing it to swell and possibly burst.

Question 11

The average osmotic pressure of seawater at 20 °C is 30 atm. Assuming the only solute in seawater is sodium chloride, what is the molarity of seawater? ($$R = 0.0821 L$$·atm/mol·K, $$T = 293 K)$$

Accepted Answer

$$\pi = iMRT$$; $$M = \frac{30}{2 	imes 0.0821 	imes 293} = 0.625 M$$

Question 12

Which of the following is NOT a colligative property?

Accepted Answer

Density increase

Question 13

Which of the following best explains why adding salt to ice causes the ice to melt at a lower temperature?

Accepted Answer

Salt lowers the freezing point of water by increasing the number of solute particles, which disrupts the formation of the solid phase.

Question 14

Which of the following equations is used to calculate the freezing point depression of a solution?

Accepted Answer

$$\Delta T_f = i K_f m$$

Question 15

A solution is prepared by dissolving a nonvolatile solute in water. Which of the following statements is true?

Accepted Answer

The vapor pressure of the solution is lower than that of pure water.

Question 16

Which of the following best describes the effect of adding a solute to a solvent on the boiling point of the solution?

Accepted Answer

The boiling point increases because the vapor pressure is lowered.

Colligative Properties of Solutions: Vapor Pressure, Boiling/Freezing Points, and Osmotic Pressure

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