**A.**

$\overline{){{\mathbf{K}}}_{{\mathbf{a}}}{\mathbf{=}}\frac{\mathbf{products}}{\mathbf{reactants}}{\mathbf{=}}\frac{\left[{H}^{+}\right]\left[{{\mathrm{NO}}_{2}}^{-}\right]}{\left[{\mathrm{HNO}}_{2}\right]}}$

The value of K_{a} for nitrous acid (HNO_{2}) at 25 °C is 4.5×10^{−4}.

a. Write the chemical equation for the equilibrium that corresponds to K_{a}

1. H^{+}(aq) + NO^{−2}(aq) ⇌ HNO_{2}(aq)

2. HNO_{2}(aq) ⇌ H^{+}(aq) + NO^{−2}(aq)

3. HNO_{2}(aq) ⇌ H^{−}(aq) + NO^{+2}(aq)

4. HNO_{2}(aq) + H^{+}(aq) ⇌ H_{2}NO^{+2}(aq)

5. HNO_{2}(aq) + H^{−}(aq) ⇌ H_{2}NO^{+2}(aq)

b. By using the value of K_{a} , calculate ΔG° for the dissociation of nitrous acid in aqueous solution.

c. What is the value of ΔG at equilibrium?

d. What is the value of ΔG when [H^{+}] = 5.9 ×10^{−2} M , [NO^{−2}] = 6.7 × 10^{−4} M , and [HNO_{2}] = 0.21 M ?

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