CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
Pure water is not always neutral at pH=7.00. Consider aqueous systems maintained at 40∘C, for which the ion–product constant of water is Kw=2.92×10−14.
Three independent samples are prepared at this temperature.
• Sample A: pH=7.00
• Sample B: [OH−]=4.0×10−7M
• Sample C: pOH=6.70
(1) Calculate the numerical value of the pH of neutral pure water at 40∘C to two decimal places.
(2) For each of the three samples (A, B, and C) determine whether the solution is acidic, basic, or neutral at 40∘C. Support each classification with appropriate quantitative evidence.
(3) For Sample B, compute [H3O+] and compare it with the value you obtained for neutral water in part (a). Based on this comparison, discuss whether the excess ions in Sample B arise primarily from water auto-ionization or from an added acid/base.