When copper reacts with nitric acid, a mixture of \(\mathrm{NO}(g)\) and
\(\mathrm{NO}_{2}(g)\) is evolved. The volume ratio of the two product gases
depends on the concentration of the nitric acid according to the equilibrium
$$2 \mathrm{H}^{+}(a q)+2 \mathrm{NO}_{3}^{-}(a q)+\mathrm{NO}(g)
\rightleftharpoons 3 \mathrm{NO}_{2}(g)+\mathrm{H}_{2} \mathrm{O}(l)$$
Consider the following standard reduction potentials at \(25^{\circ}
\mathrm{C}:\)
$$\begin{aligned}
&3 \mathrm{e}^{-}+4 \mathrm{H}^{+}(a q)+\mathrm{NO}_{3}^{-}(a q)
\longrightarrow \mathrm{NO}(g)+2 \mathrm{H}_{2} \mathrm{O}(l)\ &
\mathscr{E}^{\circ}=0.957 \mathrm{V}
\end{aligned}$$
$$\begin{aligned}
&\mathrm{e}^{-}+2 \mathrm{H}^{+}(a q)+\mathrm{NO}_{3}^{-}(a q) \longrightarrow
\mathrm{NO}_{2}(g)+2 \mathrm{H}_{2} \mathrm{O}(l) &\mathscr{E}^{\circ}=0.775
\mathrm{V}
\end{aligned}$$
a. Calculate the equilibrium constant for the above reaction.
b. What concentration of nitric acid will produce a NO and \(\mathrm{NO}_{2}\)
mixture with only \(0.20 \% \mathrm{NO}_{2}\) (by moles) at \(25^{\circ}
\mathrm{C}\) and 1.00 atm? Assume that no other gases are present and that the
change in acid concentration can be neglected.