What is the pH of a solution prepared by dissolving 0.020 mol of a strong monoprotic acid HA in enough water to make 500 mL of solution? Assume complete dissociation.
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Correct answer
B. 1.4
Principle or equation
For a strong monoprotic acid, [H+] equals the acid concentration. pH = -log[H+]. Concentration = moles/volume in liters.
Why this answer is correct
Moles of HA = 0.020 mol, volume = 0.500 L, so [HA] = 0.020/0.500 = 0.040 M. Since it is strong and monoprotic, [H+] = 0.040 M. pH = -log(0.040) ≈ 1.40, which rounds to 1.4.
Example
If 0.010 mol HCl is dissolved in 250 mL, [H+] = 0.040 M, pH = 1.40.
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