A student dissolves 0.075 mol of a strong monoprotic acid HA in enough water to make 1.50 L of solution. What is the pH of the resulting solution? Assume complete dissociation.
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Correct answer
A. 1.30
Principle or equation
For a strong monoprotic acid that fully dissociates, the concentration of H+ equals the acid concentration. pH = -log10[H+].
Why this answer is correct
First calculate the molarity of HA: 0.075 mol / 1.50 L = 0.050 M. Since HA is a strong monoprotic acid, [H+] = 0.050 M. Then pH = -log(0.050) = 1.30.
Example
If 0.020 mol of HCl is dissolved in 0.400 L, [H+] = 0.050 M, pH = 1.30.
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