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?
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:
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.
• 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)