دکتر امید رجبی دانشیار گروه شیمی دارویی شیمی عمومی

Chemistry and Calculations for the Hydrolysis of Salts

The ions of salts can have an influence on the pH of a solution. Ions that come from a strong acid or base do not influence the pH. WHY? Since strong acids and bases are 100% ionized in water, the ions are unable to reform the molecular acid or the base in water. HCl + H 2 O  H 3 O + + Cl - “one way arrow” NaOH + H 2 O  Na + + OH - + H 2 O “one way arrow” NaCl is the salt that comes from a strong acid and a strong base.

What would the pH of a sodium chloride solution 25 o C)? pH = 7 What gives rise to this pH? Auto hydrolysis of water. H 2 O + H 2 O ↔ H 3 O + + OH -

Salts that contain ions that come from a weak acid or base. weak acid: HNO 2 A salt containing the anion of the weak acid and the cation from a strong base. KNO 2 Add water: KNO 2 (s) + H 2 O  K + + NO H 2 O Hydrolysis: NO H 2 O ↔ HNO 2 + OH - A basic solution.

Calculate the pH of a 0.10 M KNO 2 solution. K a (HNO 2 ) = CHEMISTRY: KNO 2 (s)  K + + NO 2 - More Chemistry: NO H 2 O ↔ HNO 2 + OH - Equilibrium: ICEICE 0.10 N/A 0 0 -X N/A +X +X 0.10-X N/A +X +X K a K b = 1.0 x x =1.49 x = [OH - ] pOH = -log[OH - ] = 5.83 pH = 14 – 5.83 = 8.17 Try dropping

Salts that contain ions that come from a weak acid or base. Weak Base: (CH 3 ) 3 N trimethylamine A salt containing the cation of the weak base and the anion from a strong acid. (CH 3 )NHCl trimethylammonium chloride Add water: (CH 3 )NHCl (s) + H 2 O  (CH 3 )NH + + Cl - + H 2 O Hydrolysis : (CH 3 )NH + + H 2 O ↔ (CH 3 )NHOH + H + An acidic solution.

Calculate the pH of a 0.10 (CH 3 )NHCl solution. K b ((CH 3 )NHCl ) = CHEMISTRY: (CH 3 )NHCl(s) + H 2 O  (CH 3 )NH + + Cl - + H 2 O More Chemistry: (CH 3 )NH + + H 2 O ↔ (CH 3 )NHOH - + H + Equilibrium: ICEICE 0.10 N/A 0 0 -x N/A +x +x 0.10-x N/A +x +x K a K b = 1.0 x Try dropping x =3.68 x = [H + ] pH = -log[H + ] = 5.43

What if both ions of a salt come from weak acid and a weak base? Then the K a and K b of the acid or base from which the ions come from must be compared. NH 4 CN (aq) K a (NH 4 + ) = 5.6 x K b (CN - ) = 2.04 x note: K a was calculated from K b (NH 3 ) and K b from K a HCN Since K b (CN - ) is greater than K a (NH 4 + ), the solution is basic.