2. دکتر امید رجبی دانشیار گروه شیمی دارویی شیمی عمومی

3. Chemistry and Calculations for the Hydrolysis of Salts

4. 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.

5. 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 -

6. 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 2 - + H 2 O Hydrolysis: NO 2 - + H 2 O ↔ HNO 2 + OH - A basic solution.

7. Calculate the pH of a 0.10 M KNO 2 solution. K a (HNO 2 ) = 4.5 10 -4. CHEMISTRY: KNO 2 (s)  K + + NO 2 - More Chemistry: NO 2 - + 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 10 -14 x =1.49 x 10 -6 = [OH - ] pOH = -log[OH - ] = 5.83 pH = 14 – 5.83 = 8.17 Try dropping

8. 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.

9. Calculate the pH of a 0.10 (CH 3 )NHCl solution. K b ((CH 3 )NHCl ) = 7.4 10 -5. 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 10 -14 Try dropping x =3.68 x 10 -6 = [H + ] pH = -log[H + ] = 5.43

10. 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 10 -10 K b (CN - ) = 2.04 x 10 -5 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.