Question

What is the H^+ ion concentration in pure water?

(a) 1×10^7 m

(b) 1×10^-7 m

(c) 1×10^14 m

(d) 1×10^-14 m

This question was addressed to me by my college professor while I was bunking the class.

I would like to ask this question from Buffers topic in portion Introduction to Fermentation Processes & Media for Industrial Fermentation of Bioprocess Engineering

Answer 1

The correct answer is:

(b) 1×10^-7 M

Explanation: In pure water, the concentration of hydrogen ions (H⁺) is 1×10^-7 M at 25°C. This is because water undergoes autoionization, where a small fraction of water molecules dissociate into hydrogen ions (H⁺) and hydroxide ions (OH⁻):

H2O(l)⇌H+(aq)+OH−(aq)H_2O (l) \rightleftharpoons H^+ (aq) + OH^- (aq)H2​O(l)⇌H+(aq)+OH−(aq)

At 25°C, the concentrations of H⁺ and OH⁻ in pure water are equal, each being 1×10^-7 M. This results in a neutral pH of 7, as pH is defined as the negative logarithm of the hydrogen ion concentration:

pH=−log⁡[H+]\text{pH} = -\log[H^+]pH=−log[H+]

Since [H⁺] = 1×10^-7 M in pure water, the pH of pure water is 7.