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Equilibrium Free-Response Questions for AP Chemistry

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By — McGraw-Hill Professional
Updated on Feb 9, 2011

Review the following concepts if necessary:

Question

You have 15 minutes. You may use a calculator and the tables at the back of the book.

An aqueous solution is prepared that is initially 0.100 M in CdI42–. After equilibrium is established, the solution is found to be 0.013 M in Cd2+.

  1. Derive the expression for the dissociation constant, Kd, and determine the value of the constant.
  2. What will be the cadmium ion concentration arising when 0.400 mol of KI is added to 1.00 L of the solution in part a?
  3. A solution is prepared by mixing 0.500 L of the solution from part b and 0.500 L of 2.0 × 10–5 M NaOH. Will cadmium hydroxide, Cd(OH)2, precipitate? The Ksp for cadmium hydroxide is 2.2 × 10–14.
  4. When the initial solution is heated, the cadmium ion concentration increases. Is the equilibrium an exothermic or an endothermic process? Explain how you arrived at your conclusion.

Answers and Explanations

  1. Kd + [Cd2+][I]M4/[CdI42–] Give yourself 1 point for this expression. Using the following table:
  2. The value of [Cd2+] is given (= 0.013), and this is x. This changes the last line of the table to:

    Entering these values into the Kd expression gives: (0.013)(0.052)4/(0.087) = 1.1 × 10–6. Give yourself 1 point for this answer. You can also get 1 point if you correctly put your values into the wrong equation.

  3. The table in part a changes to the following:
  4. Give yourself 1 point for the correct setup and 1 point for the correct answer. If you got the wrong K in part a, you can still get one or both points for using it correctly.

  5. This value is less than Ksp, so no precipitate will form.

    Give yourself 1 point for a correct calculation, and another point for the correct conclusion. If you correctly use a wrong [Cd2+] you calculated in part b, you can still get both points.

  6. Since the cadmium ion concentration increases, the equilibrium must shift to the right. Endothermic processes shift to the right when they are heated. This is in accordance with Le Châtelier's principle.
  7. Give yourself 1 point for endothermic. Give yourself 1 point for mentioning Le Châtelier's principle.

Your score is based on a total of 8 points. Subtract one point if any answer has an incorrect number of significant figures.

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