A student prepares a solution by dissolving 0.030 mol of a strong monoprotic acid HA in enough water to make 600 mL 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, complete dissociation gives [H+] equal to the acid concentration. pH = -log10([H+]).
Why this answer is correct
First calculate molarity: 0.030 mol / 0.600 L = 0.050 mol/L. Since the acid is strong and monoprotic, [H+] = 0.050 M. Then pH = -log(0.050) = 1.301, approximately 1.30.
Example
If 0.010 mol of HCl is dissolved to make 250 mL, [H+] = 0.040 M and pH = -log(0.040) = 1.40.
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