Acid/Base Review

Worksheet 21 - Acid/Base Calculations 1. Start with 100.00 mL of a 0.10 M solution of acetic acid, CH3COOH. It has a pH = 2.87 at 25oC. Calculate the Ka of acetic acid at 25oC. a) Write the acid dissociation reaction.

b)

Write an expression for Ka, in terms of the reactants and products of this reaction.

c)

Write an ICE table for these experimental conditions.

d)

Solve for [H+] with the information in your table and equations. Then, calculate the value for Ka.

e)

Calculate the % ionization for 0.10 M acetic acid. Is the 5% approximation O.K.? [H+]eq x 100 = [CH3COOH]i

g)

Now, add 900.00 mL of water to this system. Calculate the pH of a 0.010 M solution of acetic acid, using the same Ka.

h)

Finally, calculate the % ionization. Is it larger or smaller than in the 0.10 M solution? Explain this in terms of Le Chatelier's Principle. (Hint: think about the effects of volume changes on gaseous systems)

2.

What is the pH of a 0.0010 M solution of methyl amine, CH3NH2, with a Kb = 4.4 x 10-4?

3.

A 0.1 M solution of a monoprotic acid, HA, is 4.2 % ionized. What is the Ka?

Polyprotic acids are those with more than one acidic proton. One example is arsenic acid, H3AsO4, a triprotic acid. It has three equilibrium expressions associated with its reaction with water: H3AsO4 + H2O H2AsO4- + H2O HAsO42- + H2O H2AsO4- + H3O+ HAsO42- + H3O+ AsO43- + H3O+ Ka1 = 5.0 x 10-3 Ka2 = 8.0 x 10-8 Ka3 = 6.0 x 10-10

It is a weak acid, whose ionization constants decrease markedly for the second and third dissociation reactions. The concentrations of each of the four species can be calculated as follows: The value of Ka1 will be used to calculate the concentration of H3AsO4, H2AsO4and H+. These concentrations will not be affected by the other equilibria. The value of Ka2 will be used to calculate the concentration of HAsO42-, using the previously calculated value of H2AsO4- and H3O+. The value of Ka3 will be used to calculate the...