What is the value of the water ionization constant Kw at 25°C?

What is the value of the water ionization constant Kw at 25°C?

• A. 1.0 x 10^-14
• B. 1.0 x 10^-7
• C. 1.0 x 10^-10
• D. 1.0

Answer: (A)

The value tests the definition of Kw, the ion-product of water. Kw = [H+][OH−]. In pure water at 25°C, [H+] equals [OH−], both being 1.0 × 10^−7 M. Multiplying gives Kw = (1.0 × 10^−7)^2 = 1.0 × 10^−14 (mol/L)^2. The temperature dependence matters—Kw changes with temperature, which is why the standard value quoted for 25°C is 1.0 × 10^−14. This also explains why neutral water has pH 7, since [H+] = 1.0 × 10^−7 M. The other numbers aren’t Kw: 1.0 × 10^−7 would be a single ion concentration, not the product; 1.0 × 10^−10 would correspond to much more concentrated ions; and Kw has units (mol/L)^2, not a pure number.

The value tests the definition of Kw, the ion-product of water. Kw = [H+][OH−]. In pure water at 25°C, [H+] equals [OH−], both being 1.0 × 10^−7 M. Multiplying gives Kw = (1.0 × 10^−7)^2 = 1.0 × 10^−14 (mol/L)^2. The temperature dependence matters—Kw changes with temperature, which is why the standard value quoted for 25°C is 1.0 × 10^−14. This also explains why neutral water has pH 7, since [H+] = 1.0 × 10^−7 M. The other numbers aren’t Kw: 1.0 × 10^−7 would be a single ion concentration, not the product; 1.0 × 10^−10 would correspond to much more concentrated ions; and Kw has units (mol/L)^2, not a pure number.