19. For the equilibrium that exists in an aqueous solution of nitrous acid (HNO₂, a weak acid), the equilibrium constant expression is:
(A) \( K = \frac{[\text{H}^+][\text{NO}_2^-]}{[\text{HNO}_2]} \)
(B) \( K = \frac{[\text{H}^+][\text{N}^{3+}][\text{O}^{2-}]^2}{[\text{HNO}_2]} \)
(C) \( K = [\text{H}^+][\text{NO}_2^-] \)
(D) \( K = \frac{[\text{H}^+]^2[\text{NO}_2^-]}{[\text{HNO}_2]} \)

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