Pure water at 25 degrees C has a pH of

Pure water at 25 degrees C has a pH of

• A. 1
• B. 5
• C. 7
• D. 14

Answer: (C)

Pure water at 25°C is neutral because it autoionizes into equal amounts of hydronium and hydroxide ions. The autoionization constant Kw at this temperature is 1.0 × 10^-14, so [H+] × [OH-] = 1.0 × 10^-14. If the solution is neutral, [H+] = [OH-], giving each concentration as √(Kw) = 1.0 × 10^-7 M. The pH is -log10(1.0 × 10^-7) = 7, and pOH is also 7, with pH + pOH = 14. Values like 1, 5, or 14 would imply either a much higher [H+] or [OH-], breaking the equality of the two ions, so they don’t describe pure water at this temperature.

Pure water at 25°C is neutral because it autoionizes into equal amounts of hydronium and hydroxide ions. The autoionization constant Kw at this temperature is 1.0 × 10^-14, so [H+] × [OH-] = 1.0 × 10^-14. If the solution is neutral, [H+] = [OH-], giving each concentration as √(Kw) = 1.0 × 10^-7 M. The pH is -log10(1.0 × 10^-7) = 7, and pOH is also 7, with pH + pOH = 14.

Values like 1, 5, or 14 would imply either a much higher [H+] or [OH-], breaking the equality of the two ions, so they don’t describe pure water at this temperature.