Which statement is true about Kw at a given temperature?

Which statement is true about Kw at a given temperature?

• A. Kw equals [H+] + [OH-].
• B. Kw equals [H+] [OH-] and is often given as 1 × 10^-14 at 25°C.
• C. Kw equals [H+] / [OH-].
• D. Kw equals [H+] × [OH-] squared.

Answer: (B)

Kw is the ionic product of water, the equilibrium constant for the autoionization of water. At a fixed temperature, Kw equals the product of the activities of the hydronium and hydroxide ions: Kw = a(H+) a(OH-). In dilute aqueous solutions this is well approximated by concentrations, so Kw ≈ [H+][OH-]. At 25 °C this product is about 1.0 × 10^-14, which helps explain why neutral water has equal but very small concentrations of H+ and OH- (around 1 × 10^-7 M each). The other forms aren’t correct because Kw is a product, not a sum, ratio, or something involving squaring one of the concentrations; Kw is also temperature-dependent, with the numerical value given specifically for 25 °C.

Kw is the ionic product of water, the equilibrium constant for the autoionization of water. At a fixed temperature, Kw equals the product of the activities of the hydronium and hydroxide ions: Kw = a(H+) a(OH-). In dilute aqueous solutions this is well approximated by concentrations, so Kw ≈ [H+][OH-]. At 25 °C this product is about 1.0 × 10^-14, which helps explain why neutral water has equal but very small concentrations of H+ and OH- (around 1 × 10^-7 M each). The other forms aren’t correct because Kw is a product, not a sum, ratio, or something involving squaring one of the concentrations; Kw is also temperature-dependent, with the numerical value given specifically for 25 °C.