A student dissolves 0.060 mol of a strong diprotic acid H2A in enough water to make 300.0 mL of solution. What is the pH of the resulting solution? Assume complete dissociation.
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Correct answer
A. 0.40
Principle or equation
For a strong diprotic acid, each mole of H2A produces two moles of H+ upon complete dissociation. The concentration of H+ is 2 times the acid concentration. pH = -log10[H+].
Why this answer is correct
Moles of H2A = 0.060 mol. Volume = 300.0 mL = 0.300 L. Acid concentration = 0.060 mol / 0.300 L = 0.20 M. Since H2A is diprotic, [H+] = 2 × 0.20 M = 0.40 M. pH = -log10(0.40) ≈ 0.40.
Example
For 0.050 mol H2SO4 in 250 mL, [H2SO4] = 0.20 M, [H+] = 0.40 M, pH ≈ 0.40.
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