More work with Weak Acids, Ka, and pH April 25, 2012

October 11, Second Niacin problem The K a for niacin is 1.6 x What is the pH of a M solution of niacin? 1 st find the [H + ] at equilibrium NiacinH+H+ niacin ion Initial Change-x+x Equilibrium xxx

October 11, K a = [H + ] [niacin ion] = 1.6 x [niacin] 1.6 x = x 2 / (0.010-x) x x x x = 0 x = 3.92 x = [H + ] pH = -log(3.92 x 10 –4 ) pH = 3.41

October 11, Typical behavior of weak acids The concentration of H + is only a small fraction of the concentration of the acid in solution. Relates directly to acid properties, like – Electrical conductivity – Rate of reaction with an active metal

October 11, The percent ionization of a weak acid _____________ as it concentration increases.

October 11, Check this out to see how the numbers work. Calculate the percentage of HF molecules ionized in – A. a 0.10 M HF solution – B. a M HF solution HF (aq)  H + (aq) + F - (aq) K a = 6.8 x 10 -4

October 11, HF (aq)  H + (aq) + F - (aq) I 0.10 M 0 M 0 M C - x + x + x E 0.10 – x x x Solve x 2 = 6.8 x (0.10 –x) X = 7.9 x M [H + ]

October 11, Now, for percent ionization = conc ionized x 100% original conc = 7.9 x M x 100% = 7.9% 0.10 M Repeat the steps for concentration of M: X = 2.3 x M % ionization = 23 %

October 11, How can this be? It is what we would expect from Le Chatelier’s Principle. HF (aq)  H + (aq) + F - (aq) Dilution causes the concentration of the products to decrease, To make up for that decrease, the equilibrium shifts to the right (product side). Increases the amount of ionization.

A M weak acid solution, HQ is only 4.5% ionized. Calculate the equilibrium [H + ], [Q - ], [HQ], pH, and Ka for the acid HQ. HQ  H + + Q - October 11, [H + ]=[Q - ] = M Q - [HQ] at equilibrium = – = M

Continued…. October 11, pH = -log(0.0025) = 2.6

October 11, The solubility of CO 2 in pure water at 25 0 c and 1 atm pressure is M. The common practice is to assume that all the of the dissolved CO 2 is in the form of carbonic acid (H 2 CO 3 ). CO 2 (aq) + H 2 O  H 2 CO 3 (aq) What is the pH of a M solution of H 2 CO 3 ?

October 11, H 2 CO 3 is a diprotic acid, and K a1 (4.3 x ) and K a2 (5.6 x ) differ by more than a factor of The pH can be determined by considering only K a1. H 2 CO 3 H+H+ HCO 3 1- Initial M00 Change-x+x Equilibrium xxx

October 11, K a1 = 4.3 x = [H + ][HCO 3 1- ] [H 2 CO 3 ] 4.3 x = x 2 / X 2 = (4.3 x )(0.0037) x = 3.99 x = [H + ] pH = -log (3.99 x ) = 4.40

Homework Chapter 16 Problems 43, 44, 46, 48, 50, 52, 54