2. Weak acids and Bases in water دکتر امید رجبی دانشیار گروه شیمی دارویی دانشکده داروسازی مشهد دکتر امید رجبی دانشیار گروه شیمی دارویی دانشکده داروسازی مشهد شیمی عمومی

3. Weak acids in water Arrhenius: An acid furnishes a hydronium ion (H + or H 3 O + ) in solution. Bronsted-Lowry: An acid is a proton donor HC 2 H 3 O 2 + H 2 O  C 2 H 3 O 2 - + H 3 O + acid base conj base conj. acid H 4 P 2 O 7 + H 2 O  H 3 P 2 O 7 - + H 3 O + The H + (H 3 O + ) tells us this a K a Problem or: HA  H + + A -

4. Problem: Butanoic acid, HC 4 H 7 O 2 has a K a = 1.5 x 10 - 5. Calculate the [H + ] and pH of a 0.09 M butanoic acid solution. 1st write equilibrium: HC 4 H 7 O 2  H + + C 4 H 7 O 2 - I. C. E. 0.09 0 0 -x +x +x 0.09 - x x x try dropping x = 0.00116 = [H + ] pH = -log 0.00116 = 2.93 pOH = 11.065 and the [OH - ] = 8.61 x 10 - 12

5. Weak Bases in Water: Arrhenius: A base furnishes a hydroxide ion (OH - ) in solution. NH 3 C 6 H 5 NH 2 (CH 3 ) 2 NH + H 2 O  NH 4 + + OH - + H 2 O  C 6 H 5 NH 3 + + OH - + H 2 O  (CH 3 ) 2 NH 2 + + OH - The OH - ion tells us that these are K b problems. Also: K a K b =K w and K w @ 25 o C = 1 x10 -14 Bronsted-Lowry: A base is a proton acceptor base acid conj acid conj. base

6. Given: K b for trimethylamine (CH 3 ) 3 N is 6.4 x 10 -5 Problem: Calculate the pH of a 0.05 M (CH 3 ) 3 N solution. Hydrolysis Eq. Expression: (CH 3 ) 3 N + H 2 O  (CH 3 ) 3 NH + + OH - I. C. E. 0.05 0 0 -x +x +x 0.05 - x x x try dropping to simplify the math x = 1.79 x 10 -3 What does x represent? [OH - ] pOH = -log[1.79 x 10 -3 ] = 2.75pH = 14 -2.75 = 11.25

7. Given: K b for trimethylamine (CH 3 ) 3 N is 6.4 x 10 -5 Problem: Calculate the pH of a 0.05 M (CH 3 ) 3 N solution. Hydrolysis Eq. Expression: (CH 3 ) 3 N + H 2 O  (CH 3 ) 3 NH + + OH - I. C. E. 0.05 0 0 -x +x +x 0.05 - x x x try dropping to simplify the math x = 1.79 x 10 -3 What does x represent? [OH - ]