A student prepares a solution by dissolving 0.050 mol of a strong diprotic acid H2A in enough water to make 250.0 mL of solution. What is the pH of the resulting solution? Assume complete dissociation.
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Correct answer
C. 1.00
Principle or equation
For a strong diprotic acid, complete dissociation gives two moles of H+ per mole of acid. pH = -log[H+].
Why this answer is correct
Molarity of H2A = 0.050 mol / 1.00 L = 0.050 M. [H+] = 2 × 0.050 = 0.10 M. pH = -log(0.10) = 1.00.
Example
If 0.020 mol of H2SO4 is dissolved to make 0.500 L, [H+] = 2 × (0.020/0.500) = 0.080 M, pH = 1.10.
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