Pure water undergoes autoionization according to the equilibrium
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
Pure water undergoes autoionization according to the equilibrium 2H2O(l)⇌H3O+(aq)+OH−(aq), with an equilibrium constant Kw=1.0×10−14 at 25∘C.
(a) Describe the quantitative definitions of pH and pOH and explain, in molecular terms, what each quantity measures in an aqueous solution.
(b) Derive the numerical relationship between pH and pOH at 25∘C using the value of Kw. Clearly state any assumptions made.
(c) A student claims that a sample of pure water at 25∘C must always have pH =7.00. Evaluate this claim by explaining how the autoionization constant of water changes with temperature and what effect this has on the pH of neutral water at temperatures other than 25∘C. No calculations are required, but your explanation must be quantitative and logically structured.