A 0.0100 M solution of HCl has what pH?

A 0.0100 M solution of HCl has what pH?

• A. 1.00
• B. 2.30
• C. 3.00
• D. 2.00

Answer: (D)

Strong acids dissociate completely in water, so the hydrogen ion concentration equals the acid concentration. For HCl, a monoprotic strong acid, [H+] = 0.0100 M. The pH is defined as pH = -log10([H+]). Therefore, pH = -log10(0.0100) = 2.00. The three significant figures in 0.0100 M carry through to the pH, giving 2.00. At this concentration, neglecting water’s autoionization is reasonable, so the calculation is straightforward. If the pH were 1.00 or 3.00, the corresponding [H+] would be 0.1 M or 0.001 M, respectively, which isn’t the case here; pH 2.30 would require [H+] ≈ 5 × 10^-3 M, also not matching 0.0100 M.

Strong acids dissociate completely in water, so the hydrogen ion concentration equals the acid concentration. For HCl, a monoprotic strong acid, [H+] = 0.0100 M. The pH is defined as pH = -log10([H+]). Therefore, pH = -log10(0.0100) = 2.00. The three significant figures in 0.0100 M carry through to the pH, giving 2.00. At this concentration, neglecting water’s autoionization is reasonable, so the calculation is straightforward. If the pH were 1.00 or 3.00, the corresponding [H+] would be 0.1 M or 0.001 M, respectively, which isn’t the case here; pH 2.30 would require [H+] ≈ 5 × 10^-3 M, also not matching 0.0100 M.