Q: If the equilibrium constant $$K_c$$ for a reaction is $$1.0 \times 10$$^{-5}$$ and the initial concentration of reactant is 0.10$ M, which of the following is the best justification for using the approximation method?

$$0.10 is $$much greater than $$K_c \times 400$$, so the change in concentration is negligible.

Question 1

For the reaction $$\mathrm{H_2$(g) + I_2$(g) \rightleftharpoons 2HI(g)}$$, a mixture is prepared with $$0.200$$ mol of $$\mathrm{H_2$}$$ and $$0.200$$ mol of $$\mathrm{I_2$} in a$$ $$2.00 L $$flask. At equilibrium, the concentration of $$\mathrm{I_2$} is$$ $$0.020$$ mol L$$^{-1}. $$What is the equilibrium constant $$K_c$$ for this reaction?

Accepted Answer

$$K_c = 0.064$$

Question 2

Which of the following statements about using an ICE table to solve equilibrium problems is CORRECT?

Accepted Answer

The changes in concentration always occur in the same ratio as the coefficients in the balanced equation.

Question 3

Given the reaction $$2\mathrm{H_2O(g)} ightleftharpoons 2\mathrm{H_2$(g)} + \mathrm{O_2$(g)}$$ with $$K_c = 7.3 	imes 10$$^{-18}$$ at 1000$$\ ^\circ\mathrm{C}$$, and an initial $$[\mathrm{H_2O}] = 0.100$$ M, what is the equilibrium concentration of $$\mathrm{$$H_2$$}$$?

Accepted Answer

$$5.2 	imes 10$$^{-7}$$ M

Question 4

Why is it valid to use the approximation $$0.100 - 2x \approx 0.100$$ when solving for equilibrium concentrations in the decomposition of water at $$1000\ ^\circ\mathrm{C}$$?

Accepted Answer

Because $$x is $$much smaller than $$0.100$$, so subtracting $$2x$$ does not significantly change the value.

Question 5

A reaction mixture initially contains $$0.300 M$$ $$\mathrm{A}$$ and $$0.200 M$$ $$\mathrm{B}$$ for the reaction $$\mathrm{A + B \rightleftharpoons C}. At $$equilibrium, $$[\mathrm{C}] = 0.100 M. $$What is the value of $$K_c$$?

Accepted Answer

$$K_c = 0.25$$

Question 6

Which of the following best describes the purpose of an ICE table in equilibrium calculations?

Accepted Answer

To organize initial, change, and equilibrium concentrations for all species in a reaction.

Question 7

For the reaction $$\mathrm{N_2$(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)}$$, if the initial concentrations are $$[\mathrm{N_2$}] = 1.00 M$$, $$[\mathrm{H_2$}] = 2.00 M$$, and $$[\mathrm{NH_3$}] = 0 M$$, and at equilibrium $$[\mathrm{NH_3$}] = 0.50 M$$, what is $$[\mathrm{H_2$}] at $$equilibrium?

Accepted Answer

$$1.25 M$$

Question 8

Which of the following is a valid assumption when using approximations to simplify equilibrium calculations?

Accepted Answer

The concentration from which $$x is $$subtracted must be at least 400 times greater than $$K.$$

Question 9

A student is solving for the equilibrium concentration of $$\mathrm{O_2$} in $$the reaction $$2\mathrm{H_2O(g)} \rightleftharpoons 2\mathrm{H_2$(g)} + \mathrm{O_2$(g)}$$ using an ICE table. If $$x is $$the change in $$\mathrm{O_2$}$$ concentration, what is the equilibrium concentration of $$\mathrm{H_2$}} in $$terms of $$x$$?

Accepted Answer

$$2x$$

Question 10

In a reaction where $$K_c is $$very small, which of the following is most likely TRUE about the equilibrium concentrations?

Accepted Answer

The concentrations of products will be much smaller than those of reactants.

Question 11

For the reaction $$\mathrm{A + 2B \rightleftharpoons 3C}$$, the initial concentrations are $$[\mathrm{A}] = 0.50 M$$, $$[\mathrm{B}] = 0.60 M$$, and $$[\mathrm{C}] = 0 M. At $$equilibrium, $$[\mathrm{C}] = 0.30 M. $$What is the equilibrium concentration of $$\mathrm{B}$$?

Accepted Answer

$$0.40 M$$

Question 12

Which of the following is NOT a correct step when constructing an ICE table?

Accepted Answer

Using initial concentrations in the equilibrium constant expression.

Question 13

A reaction mixture contains $$[\mathrm{X}] = 0.40 M$$, $$[\mathrm{Y}] = 0.20 M$$, and $$[\mathrm{Z}] = 0.10 M at $$equilibrium for the reaction $$\mathrm{X + Y \rightleftharpoons Z}. $$What is $$K_c$$?

Accepted Answer

$$K_c = 1.25$$

Question 14

If the equilibrium constant $$K_c$$ for a reaction is $$1.0 	imes 10$$^{-5}$$ and the initial concentration of reactant is 0.10$ M, which of the following is the best justification for using the approximation method?

Accepted Answer

$$0.10 is $$much greater than $$K_c 	imes 400$$, so the change in concentration is negligible.

Question 15

In the reaction $$\mathrm{A + B \rightleftharpoons C}$$, if the initial concentrations are $$[\mathrm{A}] = 0.50 M$$, $$[\mathrm{B}] = 0.50 M$$, and $$[\mathrm{C}] = 0 M$$, and $$K_c = 1.0$$, what is the equilibrium concentration of $$\mathrm{C}$$?

Accepted Answer

$$0.29 M$$

Question 16

Which of the following is a common mistake when using ICE tables?

Accepted Answer

Using initial concentrations in the equilibrium constant expression.

Question 17

A reaction has $$K_c = 2.0 at a $$certain temperature. If the equilibrium concentrations are $$[\mathrm{A}] = 0.20 M$$, $$[\mathrm{B}] = 0.10 M$$, and $$[\mathrm{C}] = 0.04 M $$for the reaction $$\mathrm{A + B \rightleftharpoons C}$$, is the system at equilibrium?

Accepted Answer

Yes, because $$Q_c = 2.0$$ and matches $$K_c.$$

Question 18

For the reaction $$\mathrm{2A \rightleftharpoons B + C}$$, if the initial concentration of $$\mathrm{A} is$$ $$0.80 M $$and $$K_c = 0.10$$, what is the equilibrium concentration of $$\mathrm{A} ($$rounded to two decimal places)?

Accepted Answer

$$0.73 M$$

Chemical Equilibrium Calculations: Using Equilibrium Constants and ICE Tables

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