What is the pH of a 0.00200 M HClO₄solution?

Question

Quizplus · Accepted Answer

HClO is a weak acid, so it undergoes partial ionization in water, producing H+ and ClO-. The equilibrium constant expression for this reaction is Ka=[H+][ClO-]/[HClO]. 
The dissociation constant for HClO is Ka=3.0×10^-8.
To solve, set up an ICE (Initial, Change, Equilibrium) table:

HClO(aq) + H2O(l) ⇌ H3O+(aq) + ClO-(aq)

I: 0.00200 M 0 M 0 M 0 M 
C: -x 0 x x 
E: 0.00200 - x 0 x x

Plug in the equilibrium concentrations into the expression for Ka and solve for x:

Ka=[H+][ClO-]/[HClO]
3.0×10^-8 = x^2/(0.0020 - x)
Assuming x is much smaller than 0.00200, we can approximate 0.00200 - x ≈ 0.00200, which simplifies the math.

3.0×10^-8 = x^2/0.0020
x = √(3.0×10^-8 × 0.0020) = 1.94×10^-4

pH = -log[H+] = -log(x) = -log(1.94×10^-4) = 2.699, so the answer is (C).

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