If the hydrogen ion concentration is 1.0×10^-3 M, what is the solution's pH?

If the hydrogen ion concentration is 1.0×10^-3 M, what is the solution's pH?

• A. 3
• B. 11
• C. 7
• D. 0

Answer: (A)

The pH of a solution is the negative base-10 logarithm of the hydrogen ion concentration: pH = -log10[H+]. If [H+] = 1.0 × 10^-3 M, then log10[H+] = log10(1.0 × 10^-3) = -3, so pH = -(-3) = 3. This is acidic, since any pH below 7 indicates acidity, and the smaller the pH, the stronger the acidity. The pH scale is logarithmic, so a tenfold increase in [H+] lowers the pH by 1 unit, and a tenfold decrease raises it by 1 unit. For comparison, neutral water is around pH 7, and higher pH values (like 11) indicate basic solutions. Here the concentration is much greater than neutral water’s, leading to a pH of 3.

The pH of a solution is the negative base-10 logarithm of the hydrogen ion concentration: pH = -log10[H+]. If [H+] = 1.0 × 10^-3 M, then log10[H+] = log10(1.0 × 10^-3) = -3, so pH = -(-3) = 3. This is acidic, since any pH below 7 indicates acidity, and the smaller the pH, the stronger the acidity. The pH scale is logarithmic, so a tenfold increase in [H+] lowers the pH by 1 unit, and a tenfold decrease raises it by 1 unit. For comparison, neutral water is around pH 7, and higher pH values (like 11) indicate basic solutions. Here the concentration is much greater than neutral water’s, leading to a pH of 3.