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

Chemistry

Alyson Gill

13 April, 12:48

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?

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Answers (1)

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Arely Park · Answer 1

pH = 1.95

Explanation:

For polyprotic acids, it is generally assumed that all H⁺ comes from the 1st ionization step. The amount of H⁺ delivered into solution for the 2nd and 3rd ionization steps are in the order of 10⁻⁴M and 10⁻⁶M respectively and provide very little change in pH from the quantity delivered in the 1st ionization step.

Therefore ... the [H⁺] concentraion and pH are computed as follows ...

[H⁺] = √Ka₁[H₃AsO₄] = √ (2.5 x 10⁻⁴) (0.500) M = 0.1118M

pH = - log[H⁺] = - log (0.01118) = 1.95