# 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

###### FREE Expert Solution
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###### FREE Expert Solution

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)

98% (412 ratings)
###### 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

What scientific concept do you need to know in order to solve this problem?

Our tutors have indicated that to solve this problem you will need to apply the Polyprotic Acid concept. You can view video lessons to learn Polyprotic Acid. Or if you need more Polyprotic Acid practice, you can also practice Polyprotic Acid practice problems.

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Based on our data, we think this problem is relevant for Professor Keene's class at MERCER.