A solution is prepared by dissolving 0.10 mol of a strong monoprotic acid HA in enough water to make 500 mL of solution. What is the pH of the resulting solution?
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Correct answer
A. 0.70
Principle or equation
For a strong monoprotic acid, complete dissociation gives [H+] equal to the initial acid concentration. pH = -log10[H+]. Concentration is amount of solute divided by volume in liters.
Why this answer is correct
Calculate the molarity: 0.10 mol / 0.500 L = 0.20 mol/L. Since HA is a strong acid, [H+] = 0.20 M. Then pH = -log(0.20) ≈ 0.70.
Example
If 0.050 mol of HCl is dissolved in 250 mL, molarity = 0.050/0.250 = 0.20 M, pH = 0.70.
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