A student claims that the pH of an aqueous solution can never be negative

CHEMISTRY · Single correct · A Level · Chemistry · Acids and bases · Acids and bases

Problem

A student claims that the pH of an aqueous solution can never be negative. Which of the following statements most directly disproves the student’s claim? (a) A 1.0 M solution of HCl\text{HCl} has [H3O+]=1.0 M[H_3O^+] = 1.0\,\text{M}, giving a pH of 0.00. (b) A 2.5 M solution of HCl\text{HCl} has [H3O+]=2.5 M[H_3O^+] = 2.5\,\text{M}, giving a pH of about –0.40. (c) Pure water at 25 ∘C25\,^{\circ}\text{C} auto-ionizes to produce [H3O+]=1.0×10−7 M[H_3O^+] = 1.0\times10^{-7}\,\text{M}, corresponding to pH 7.00. (d) A 10 M solution of HCl\text{HCl} has [H3O+]=10 M[H_3O^+] = 10\,\text{M}, so its pH is 1.00.

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