Problem: 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 x 10-4 , K 2 = 5.6 x 10-8 , and K3 = 3 x 10-13 . What is the pH of a 0.500 M solution of arsenic acid?A) 0.85B) 1.95C) 3.90D) 4.51

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We’re being asked to determine the pH of a 0.500 M solution of arsenic acid

Recall that pH can be calculated from a solution using the equation:

pH=-log H+


Arsenic acid (H3AsO4) is a weak oxyacid with only one oxygen greater than hydrogen. H3AsO4 is a triprotic acid, meaning it can donate three protons (H+)

It has three K values and it will have three equilibrium reactions.


H3AsO4acidic form K1 H2AsO4-intermediate #1K2 HAsO42-intermediate #2K3 AsO43-basic form

We’re finding the pH of a 0.500 M solution of arsenic acid (H3AsO4) so we’re going to use the 1st equilibrium reaction and K1.

Arsenic acid will react with water:  H3AsO4(aq) + H2O(l)  H2AsO4-(aq) + H3O+(aq)

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Problem Details

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 x 10-4 , K 2 = 5.6 x 10-8 , and K3 = 3 x 10-13 . What is the pH of a 0.500 M solution of arsenic acid?

A) 0.85

B) 1.95

C) 3.90

D) 4.51

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