A student dissolves 0.0120 mol of a strong monoprotic acid HA in enough water to make 400.0 mL of solution. What is the pH of the resulting solution? Assume complete dissociation.
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Correct answer
A. 1.52
Principle or equation
For a strong monoprotic acid, the concentration of H+ equals the acid concentration. pH = -log10[H+].
Why this answer is correct
First, calculate the molarity of HA: 0.0120 mol / 0.4000 L = 0.0300 M. Since HA is a strong monoprotic acid, [H+] = 0.0300 M. Then pH = -log10(0.0300) = 1.5229, which rounds to 1.52.
Example
If 0.010 mol of HCl is dissolved to make 500 mL, [H+] = 0.020 M, pH = -log(0.020) = 1.70.
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