A solution is prepared by dissolving 0.0050 mol of a strong monoprotic acid HA in enough water to make 250 mL of solution. What is the pH of the resulting solution? Assume complete dissociation.
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Correct answer
A. 1.70
Principle or equation
For a strong monoprotic acid, [H+] = concentration of the acid. pH = -log[H+]. Concentration = moles / volume in liters.
Why this answer is correct
Volume = 250 mL = 0.250 L. [HA] = 0.0050 mol / 0.250 L = 0.020 M. Since HA is a strong acid, [H+] = 0.020 M. pH = -log(0.020) = 1.70 (since log(0.02) = -1.699).
Example
If 0.010 mol HCl is dissolved in 500 mL, [H+] = 0.020 M, pH = 1.70.
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