A student dissolves 0.0150 mol of a strong monoprotic acid HA in enough water to make 300.0 mL of solution. What is the pH of the resulting solution? Assume complete dissociation.
Show the answer and explanation
Correct answer
A. 1.30
Principle or equation
For a strong monoprotic acid that fully dissociates, the concentration of hydrogen ions equals the acid concentration. pH is defined as the negative base-10 logarithm of the hydrogen ion concentration: pH = -log10[H+]. The acid concentration is calculated as moles of solute divided by the volume of solution in liters.
Why this answer is correct
First, convert the volume from milliliters to liters: 300.0 mL = 0.3000 L. Then calculate the molarity of the acid: [HA] = 0.0150 mol / 0.3000 L = 0.0500 mol/L. Since HA is a strong monoprotic acid, it dissociates completely: HA -> H+ + A-. Thus, [H+] = 0.0500 M. Now compute the pH: pH = -log10(0.0500). Because 0.0500 = 5.00 × 10^-2, log10(0.0500) = log10(5.00) + log10(10^-2) = 0.699 - 2 = -1.301. Therefore, pH = -(-1.301) = 1.301, which rounds to 1.30.
Example
If 0.0200 mol of HCl is dissolved in 400.0 mL of water, the concentration is 0.0200 mol / 0.4000 L = 0.0500 M, so pH = -log10(0.0500) = 1.30.
This published item includes a stored explanation and passed the platform’s publication workflow. It is independent preparation material, not a claim of an official or recalled examination question.