AP Chemistry 7.3: Q, K, and Equilibrium Expressions

Practice AP Chemistry questions on writing equilibrium expressions for concentrations or partial pressures, calculating Q, and using particulate diagrams to represent equilibrium.

Exam points

Explain how equilibrium constants and reaction quotients describe composition in reversible chemical systems.
Apply concentration, pressure and temperature changes to predict equilibrium shifts and reaction outcomes.

[Maximum number: 2]

Answer the following questions about magnesium.

(a)

The dissolution of magnesium hydroxide is represented by the following equation.

\mathrm{Mg}(\mathrm{OH})_{2}(s) \rightleftarrows \mathrm{Mg}^{2+}(a q)+2 \mathrm{OH}^{-}(a q) \quad K_{s p}=5.61 \times 10^{-12}

[ 2 ]

(i)

Write the expression for the solubility product constant, $K_{s p}$ .

[ 1 ]

E (i) For the correct expression:

$K_{s p}=\left[\mathrm{Mg}^{2+}\right]\left[\mathrm{OH}^{-}\right]^{2}$

(ii)

After the two solutions are combined in beaker 3 as described in part D, but before any

reaction takes place, $\left[\mathrm{OH}^{-}\right]=1.65 \times 10^{-4} M$ . Using your answer to part D, calculate the

value of the reaction quotient, Q.

[ 1 ]

(ii) For the correct calculated value, consistent with part D and part E (i):

$Q=\left[\mathrm{Mg}^{2+}\right]\left[\mathrm{OH}^{-}\right]^{2}=\left(7.62 \times 10^{-4}\right)\left(1.65 \times 10^{-4}\right)^{2}=2.07 \times 10^{-11}$

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7.3.A—Represent the reaction quotient Qc or Qp, for a reversible reaction, and the corresponding equilibrium… question 1

Question (a)

Question (i)

Question (ii)