Predicting Predicting the pH the pH of salt of salt solutions solutions Hydrolysis of ions Hydrolysis of ions Hydrolysis refers to a reaction with water (e.g. splitting water into H + and OH – ) When salts are added to water, pH can change E.g. when Na 3 PO 4 is added to water, ions form Na 3 PO 4 (aq) → 3Na + (aq) + PO 4 3– (aq) These ions may react with H 2 O, affecting the pH PO 4 3– (aq) + H + (aq) ↔ HPO 4 2 – (aq) Na + (aq) + OH – (aq) ↔ NaOH (aq) If the anion (-ve) reacts to remove lots of H + but the cation (+ve) removes very little OH – , then H + will decrease and the solution will be basic. The degree of hydrolysis The degree of hydrolysis PO 4 3– (aq) + H + (aq) ↔ HPO 4 2 – (aq) Na + + OH – ↔ NaOH • The problem with writing equilibria this way is we do not know the strength of the reactions • However, if we reverse the reaction we can look up Ka and Kb values (pg. 608, 615): HPO 4 2 – ↔ PO 4 3– + H + NaOH ↔ Na + + OH – Ka= 4.5 x 10 – 13
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