A student dissolves 0.0200 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. 1.30
Principle or equation
pH = -log[H+]. For a strong monoprotic acid that fully dissociates, [H+] equals the acid concentration, which is moles of acid divided by volume in liters.
Why this answer is correct
Moles of HA = 0.0200 mol. Volume = 400.0 mL = 0.400 L. Concentration = 0.0200 / 0.400 = 0.0500 M. Since HA is strong and monoprotic, [H+] = 0.0500 M. pH = -log(0.0500) = 1.301, which rounds to 1.30.
Example
If 0.0100 mol of HCl is dissolved to make 250.0 mL, concentration = 0.0100/0.250 = 0.0400 M, pH = -log(0.0400) = 1.40.
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