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16.4 Calculations Involving Colligative Properties

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Chemistry 16.4

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Calculations Involving Colligative Properties

Cooking instructions often call for the addition of a small amount of salt to the cooking water. Dissolved salt elevates the boiling point of water. You will learn how to calculate the amount the boiling point of the cooking water rises.

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Calculations Involving Colligative Properties

>

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16.4 Molality and Mole Fraction

Molality and Mole Fraction

What are two ways of expressing the concentration of a solution?

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Calculations Involving Colligative Properties

> Molality and Mole Fraction

The unit molality and mole fractions are two additional ways in which chemists express the concentration of a solution.

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Calculations Involving Colligative Properties

> Molality and Mole Fraction

The unit molality (m) is the number of moles of solute dissolved in 1 kilogram (1000 g) of solvent. Molality is also known as molal concentration.

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Calculations Involving Colligative Properties

> Molality and Mole Fraction

To make a 0.500m solution of NaCl, use a balance to measure 1.000 kg of water and add 0.500 mol (29.3 g) of NaCl.

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Calculations Involving Colligative Properties

> Molality and Mole Fraction

Ethlylene Glycol (EG) is added to water as antifreeze.

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SAMPLE PROBLEM

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Practice Problems

Practice ProblemsFor Sample Problem 16.6

Problem Solving 16.29 Solve Problem 29 with the help of an interactive guided tutorial.

for Sample Problem 16.6

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Calculations Involving Colligative Properties

> Molality and Mole Fraction

The mole fraction of a solute in a solution is the ratio of the moles of that solute to the total number of moles of solvent and solute.

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Calculations Involving Colligative Properties

> Molality and Mole Fraction

In a solution containing nA mol of solute A and nB mol of solvent B (XB), the mole fraction of solute A (XA) and the mole fraction of solvent B (XB) can be expressed as follows.

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SAMPLE PROBLEM

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Sample Problem 16.7

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Practice Problems

Problem Solving 16.32 Solve Problem 32 with the help of an interactive guided tutorial.

for Practice Problem 16.7

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Calculations Involving Colligative Properties

> Freezing-Point Depression and Boiling-Point Elevation

Freezing-Point Depression and Boiling-Point Elevation

How are freezing-point depression and boiling-point elevation related to molality?

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Calculations Involving Colligative Properties

> Freezing-Point Depression and Boiling-Point Elevation

The magnitudes of the freezing-point depression and the boiling-point elevation of a solution are directly proportional to the molal concentration (m), when the solute is molecular, not ionic.

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Calculations Involving Colligative Properties

> Freezing-Point Depression and Boiling-Point Elevation

The constant, Kf, is the molal freezing-point depression constant, which is equal to the change in freezing point for a 1-molal solution of a nonvolatile molecular solute.

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Calculations Involving Colligative Properties

> Freezing-Point Depression and Boiling-Point Elevation

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Calculations Involving Colligative Properties

> Freezing-Point Depression and Boiling-Point Elevation

The constant, Kb, is the molal boiling-point elevation constant, which is equal to the change in boiling point for a 1-molal solution of a nonvolatile molecular solute.

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Calculations Involving Colligative Properties

> Freezing-Point Depression and Boiling-Point Elevation

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Calculations Involving Colligative Properties

> Freezing-Point Depression and Boiling-Point Elevation

Simulation 21 Discover the principle underlying the colligative properties of solutions.

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Calculations Involving Colligative Properties

> Freezing-Point Depression and Boiling-Point Elevation

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Practice Problems

Problem Solving 16.33 Solve Problem 33 with the help of an interactive guided tutorial.

for Sample Problem 16.8

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SAMPLE PROBLEM

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Practice Problems

Problem Solving 16.36 Solve Problem 36 with the help of an interactive guided tutorial.

for Sample Problem 16.9

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Section Quiz

-or-Continue to: Launch:

Assess students’ understanding of the concepts in Section

16.4 Section Quiz.

16.4.

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16.4 Section Quiz.

1. What is the mole fraction of He in a gaseous solution containing 4.0 g of He, 6.5 g of Ar, and 10.0 g of Ne?

a. 0.60

b. 1.5

c. 0.20

d. 0.11

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16.4 Section Quiz.

2. The freezing point depression caused by a given concentration of a nonvolatile molecular solute

a. depends on the solute.

b. depends on the solvent.

c. is always the same.

d. cannot be determined.

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16.4 Section Quiz.

3. What are the freezing and boiling points of a 0.1m solution of CaCl2 in water?

a. -0.2°C, 100.1°C

b. -0.6°C, 100.1°C

c. -0.6°C, 100.2°C

d. -0.6°C, 99.8°C

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16.4 Section Quiz.

4. Compared to the freezing point depression by ethylene glycol (C2H6O2,) for a given solvent, the freezing point depression caused by the same molal concentration of CaCl2 would be

a. exactly the same.

b. twice as large.

c. three times as large.

d. four times as large

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