A student dissolves 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, each mole of H2A produces 2 moles of H+ upon complete dissociation. The concentration of H+ is 2 × (moles of acid / volume in liters). pH = -log10[H+].
Why this answer is correct
Moles of H2A = 0.0200 mol, volume = 0.200 L. Moles of H+ = 2 × 0.0200 = 0.0400 mol. [H+] = 0.0400 / 0.200 = 0.200 M. pH = -log(0.200) = 0.699 ≈ 0.70.
Example
If 0.0100 mol of H2SO4 is dissolved in 0.100 L, [H+] = 2×0.0100/0.100 = 0.200 M, pH = 0.70.
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