Farmers who raise cotton once used arsenic acid, H₃AsO₄, as a defoliant at harvest time. Arsenic acid is a polyprotic acid with K₁ = 2.5 × 10⁻⁴, K₂ = 5.6 × 10⁻⁸, and K₃ = 3 × 10⁻¹³. What is the pH of a 0.500 M solution of arsenic acid?

Question

Quizplus · Accepted Answer

Given the values of the dissociation constants (K_a1, K_a2, K_a3) for arsenic acid, the first dissociation is much stronger than the subsequent ones. Therefore, the pH primarily depends on the first dissociation. The pH can be approximated using the formula for a weak acid: pH = 1/2(pKa - log[C]), where pKa = -log(Ka) and [C] is the concentration of the acid. Using K_a1 = 2.5 × 10^-3, pKa1 ≈ 2.6, and [C] = 0.500 M, the pH ≈ 1/2(2.6 - log(0.500)) ≈ 1.96.

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

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