At 25°C, what is the pH of pure water?

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Multiple Choice

At 25°C, what is the pH of pure water?

Explanation:
Pure water at a set temperature has its pH set by the autoionization of water: H2O ⇌ H+ + OH-. At 25°C the equilibrium constant Kw equals 1.0 × 10^-14, so [H+] = [OH-] = sqrt(Kw) = 1.0 × 10^-7 M. The pH is −log[H+], giving pH = 7, which is neutral. In real life, dissolved CO2 from the air can form carbonic acid and push the pH below 7, but when we refer to pure water under standard lab conditions (no dissolved CO2), the pH is 7. The other options describe extremes or real-world conditions that don’t apply to ideal pure water at this temperature.

Pure water at a set temperature has its pH set by the autoionization of water: H2O ⇌ H+ + OH-. At 25°C the equilibrium constant Kw equals 1.0 × 10^-14, so [H+] = [OH-] = sqrt(Kw) = 1.0 × 10^-7 M. The pH is −log[H+], giving pH = 7, which is neutral.

In real life, dissolved CO2 from the air can form carbonic acid and push the pH below 7, but when we refer to pure water under standard lab conditions (no dissolved CO2), the pH is 7. The other options describe extremes or real-world conditions that don’t apply to ideal pure water at this temperature.

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