Solutions and Colligative Properties: Free-Response Questions for AP Chemistry

Review the following concepts if necessary:

• Concentration Units for AP Chemistry
• Electrolytes and Nonelectrolytes for AP Chemistry
• Colligative Properties for AP Chemistry
• Colloids for AP Chemistry

Questions

First Free-Response Question

You have 20 minutes to answer the following questions. You may not use a calculator. You may use the tables at the back of the book.

Five beakers each containing 100.0 mL of an aqueous solution are placed on a lab bench. The solutions are all at 25°C. Solution 1 contains 0.20 M KNO₃. Solution 2 contains 0.10 M BaCl₂. Solution 3 contains 0.15 M C₂H₄(OH)₂. Solution 4 contains 0.20 M (NH₄)₂SO₄. Solution 5 contains 0.25 M KMnO₄.

1. One of the solutions could oxidize two of the other solutions. Which solution is it? Which solutions could it oxidize?
2. Which solution has the lowest pH? Explain.
3. Which pair of solutions would give a precipitate if they are mixed? What is the formula for this precipitate?
4. Which solution would be the poorest conductor of electricity? Explain.
5. Rank the solutions in order of increasing boiling point. Explain.

Second Free-Response Question

You have 20 minutes to answer the following questions. You may not use a calculator. You may use the tables in the back of the book.

Five beakers are placed in a row on a countertop. Each beaker is half filled with a 0.20 M aqueous solution. The solutes, in order, are: (1) potassium sulfate, (2) methyl alcohol, (3) sodium carbonate, (4) ammonium chromate, and (5) barium chloride. The solutions are all at 25°C.

Answer the following questions with respect to the five solutions listed above.

1. Which solution will form a precipitate when ammonium chromate is added to it? Give the formula of the precipitate.
2. Which solution is the most basic? Explain.
3. Which solution will exhibit the lowest boiling-point elevation? Explain.
4. Which solution is colored?
5. Which of the other solutions will not react with solution (5), barium chloride?

Answers and Explanations

First Free-Response Question

1. 1 point for picking solution 5, and 1 point for picking solutions 2 and 3.
2. 1 point for picking solution 4, and 1 point for saying the ammonium ion (NH₄⁺) is a weak acid, or that it undergoes hydrolysis.
3. 1 point for picking solutions 2 and 4, and 1 point for BaSO₄.
4. 1 point for picking solution 3, and 1 point for saying it is a non-electrolyte or that it does not ionize.
5. 1 point for the order: 3 < 2 < 1 < 5 < 4, and 1 point for the explanation. The explanation must relate the order to the number (concentration) of solute particles in the solution.

There is a total of 10 points possible.

Second Free-Response Question

1. Solution (5) barium chloride will give a precipitate. The formula of the precipitate is BaCrO₄.

You get 1 point for picking the correct solution, and 1 point for the correct formula for the precipitate.

2. Solution (3) sodium carbonate is the most basic. Since the carbonate ion is the conjugate base of a weak acid, it will undergo significant hydrolysis to produce a basic solution.

You get 1 point for picking the correct solution, and 1 point for the correct explanation.

3. Solution (2) methyl alcohol will show the least boiling point elevation. Methyl alcohol is the only nonelectrolyte.

You get 1 point for picking the correct solution, and 1 point for the correct formula for the explanation.

4. Solution (4) ammonium chromate is yellow.

You get 1 point for picking the correct solution.

5. Solution (2) methyl alcohol is the only solution that will not form a precipitate with barium chloride.

You get 1 point for picking the correct solution.