Farmers who raise cotton once used arsenic acid, H₃AsO₄, as a defoliant at harvest time. Arsenic acid is a polyprotic acid with K_a1 = 2.5 × 10^-4, K_a2 = 5.6 × 10^-8, and K_a3 = 3 × 10^-13. What is the pH of a 0.500 M solution of arsenic acid?

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The pH of a 0.500 M solution of arsenic acid can be approximated by considering only the first dissociation, as subsequent dissociations contribute significantly less to the concentration of H+ ions. Using the formula for the dissociation constant, $K_{a1} = \frac{[H^+][A^-]}{[HA]}$, and assuming $x \approx [H^+] \approx [A^-]$ and $[HA] \approx 0.500 - x \approx 0.500$, we can solve for $x$ using $K_{a1} = 2.5 \times 10^{-4}$. This approximation simplifies the calculation and leads to a pH close to 1.95, considering the logarithmic nature of the pH scale and the concentration of H+ ions derived from the first dissociation.

Farmers Who Raise Cotton Once Used Arsenic Acid, H3AsO4, as a Defoliant

Farmers Who Raise Cotton Once Used Arsenic Acid, H₃AsO₄, as a Defoliant