What is the pH of a 0.01 M solution of a strong acid at 25°C?

• A. pH = 2
• B. pH = 3
• C. pH = 1
• D. pH = 7

Answer: (A)

When a strong acid dissolves in water, it dissociates completely, so the hydrogen ion concentration [H+] equals the acid’s molarity. The pH is defined as the negative base-10 logarithm of [H+], pH = -log10([H+]). For a 0.01 M strong acid, [H+] = 0.01 M, and pH = -log10(0.01) = 2. This uses the 25°C standard, where the pH scale is tied to that log relation. The other values would require different [H+]: pH 3 would mean [H+] = 1×10^-3 M, pH 1 would mean [H+] = 0.1 M, and pH 7 would reflect neutral water with very little added acid. Since the acid is 0.01 M and fully dissociates, the correct pH is 2.