Ph And Acids/bases

Question

What is the pH of a 0.01 M solution of HCl, a strong acid that completely dissociates in water at 25°C?

RankGuru · Accepted Answer

pH is defined as the negative logarithm (base 10) of the hydrogen ion concentration: pH = -log[H^+]. For a strong acid like HCl, complete dissociation means [H^+] = [HCl] = 0.01 M = 10^{-2} M. Therefore, pH = -log(10^{-2}) = 2. Option 1 is wrong because pH = 1 corresponds to [H^+] = 0.1 M (i.e., 0.1 M HCl), which is 10 times more concentrated than the given solution. Option 12 is incorrect because pH 12 corresponds to a basic solution ([OH^-] = 10^{-2} M), not an acid; it might arise from confusing pOH with pH. Option 0.01 is a direct copy of the concentration value and is dimensionally incorrect since pH is a dimensionless logarithmic quantity. This is a foundational NCERT Equilibrium concept tested in every JEE Main paper. Plausibility check: for strong acids with concentration between 10^{-1} and 10^{-7} M, pH should be a positive value less than 7 at 25°C, and pH = 2 satisfies this condition.

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