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:

$\overline{){\mathbf{p}}{\mathbf{H}}{\mathbf{=}}{\mathbf{-}}{\mathbf{l}}{\mathbf{o}}{\mathbf{g}}{\mathbf{}}\left[{\mathbf{H}}^{\mathbf{+}}\right]}$

Arsenic acid (H_{3}AsO_{4}) is a **weak oxyacid** with only one oxygen greater than hydrogen. H_{3}AsO_{4} is a triprotic acid, meaning it can donate three protons (H^{+}).

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

$\stackrel{\mathbf{a}\mathbf{c}\mathbf{i}\mathbf{d}\mathbf{i}\mathbf{c}\mathbf{}\mathbf{f}\mathbf{o}\mathbf{r}\mathbf{m}}{\stackrel{\mathbf{\u23de}}{{\mathbf{H}}_{\mathbf{3}}\mathbf{A}\mathbf{s}{\mathbf{O}}_{\mathbf{4}}}}\mathbf{}\underset{{\mathbf{K}}_{\mathbf{1}}}{\mathbf{\rightleftharpoons}}\mathbf{}\stackrel{\mathbf{i}\mathbf{n}\mathbf{t}\mathbf{e}\mathbf{r}\mathbf{m}\mathbf{e}\mathbf{d}\mathbf{i}\mathbf{a}\mathbf{t}\mathbf{e}\mathbf{}\mathbf{\#}\mathbf{1}}{\stackrel{\mathbf{\u23de}}{{\mathbf{H}}_{\mathbf{2}}\mathbf{A}\mathbf{s}{\mathbf{O}}_{\mathbf{4}}^{\mathbf{-}}}}\underset{{\mathbf{K}}_{\mathbf{2}}}{\mathbf{\rightleftharpoons}}\mathbf{}\stackrel{\mathbf{i}\mathbf{n}\mathbf{t}\mathbf{e}\mathbf{r}\mathbf{m}\mathbf{e}\mathbf{d}\mathbf{i}\mathbf{a}\mathbf{t}\mathbf{e}\mathbf{}\mathbf{\#}\mathbf{2}}{\stackrel{\mathbf{\u23de}}{\mathbf{H}\mathbf{A}\mathbf{s}{\mathbf{O}}_{\mathbf{4}}^{\mathbf{2}\mathbf{-}}}}\underset{{\mathbf{K}}_{\mathbf{3}}}{\mathbf{\rightleftharpoons}}\mathbf{}\stackrel{\mathbf{b}\mathbf{a}\mathbf{s}\mathbf{i}\mathbf{c}\mathbf{}\mathbf{f}\mathbf{o}\mathbf{r}\mathbf{m}}{\stackrel{\mathbf{\u23de}}{\mathbf{A}\mathbf{s}{\mathbf{O}}_{\mathbf{4}}^{\mathbf{3}\mathbf{-}}}}$

• We’re finding the pH of a 0.500 M solution of arsenic acid (H_{3}AsO_{4}) so we’re going to use the **1 ^{st} equilibrium reaction and K_{1}**.

• *Arsenic acid will react with water: ***H _{3}AsO_{4(aq)} + H_{2}O_{(l)} ⇌ H_{2}AsO_{4}^{-}_{(aq)} + H_{3}O^{+}_{(aq)}**

Farmers who raise cotton once used arsenic acid, H_{3}AsO_{4} , as a defoliant at harvest time. Arsenic acid is a polyprotic acid with K_{1} = 2.5 x 10^{-4} , K _{2} = 5.6 x 10^{-8} , and K_{3} = 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

Frequently Asked Questions

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.

What is the difficulty of this problem?

Our tutors rated the difficulty of*Farmers who raise cotton once used arsenic acid, H3AsO4 , as...*as high difficulty.

How long does this problem take to solve?

Our expert Chemistry tutor, Dasha took 8 minutes and 18 seconds to solve this problem. You can follow their steps in the video explanation above.

What professor is this problem relevant for?

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