A student prepares a solution by dissolving 0.0200 mol of a strong diprotic acid H2A in enough water to make 200.0 mL of solution. What is the pH of the resulting solution? Assume complete dissociation.
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Correct answer
A. 0.70
Principle or equation
For a strong diprotic acid that dissociates completely, each mole of H2A produces 2 moles of H+. The concentration of H+ is twice the acid concentration. pH = -log10[H+].
Why this answer is correct
Moles of H+ = 0.0200 mol × 2 = 0.0400 mol. Volume = 200.0 mL = 0.200 L. [H+] = 0.0400 mol / 0.200 L = 0.200 M. pH = -log(0.200) = 0.699, which rounds to 0.70.
Example
For 0.0100 mol of H2SO4 in 100.0 mL, [H+] = 0.0200 mol / 0.100 L = 0.200 M, pH = 0.70.
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