# Chapter 15 - Key Skills Problems - Page 705: 15.1

(e) $[H^+] = 1.1 \times 10^{-5} \space M$

#### Work Step by Step

1. Write the equilibrium constant expression: - The exponent of each concentration is equal to its balance coefficient. $$K_c = \frac{[Products]}{[Reactants]} = \frac{[ H^+][ CN^- ]}{[ HCN ]}$$ 2. At equilibrium, these are the concentrations of each compound: $[ HCN ] = 0.25 \space M - x$ $[ H^+ ] = x$ $[ CN^- ] = x$ $$4.9 \times 10^{-10} = \frac{(x)(x)}{(0.25 - x)}$$ $x = 1.1 \times 10^{-5}$ 3. $[H^+] = 1.1 \times 10^{-5}$

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