Chemistry 112

Using Ka

If an equilibrium constant is known, then the equilibrium concentrations of products and reactants can be determined. Usually, for weak acids, this means finding the pH of the solution.

This is a standard equilibrium problem, so we follow the typical procedures.


Examples

Find the pH of a 0.010 M solution of hypochlorous acid at 25 oC.

What is a reasonable answer to expect?

If this were a strong acid, the pH would be –log (0.010) = 2.00. If this were pure water, the pH would be 7.00. Thus, our answer must lie between these values.

What is the pH of a 3.8×10–5 M solution of acetic acid at 25 oC. pKa = 4.77

What is a reasonable answer to expect?

If this were a strong acid, the pH would be –log (3.8×10–5) = 4.42. If this were pure water, the pH would be 7.00. Thus, our answer must lie between these values. Further, since Ka is fairly large, the final answer probably is a little closer to 4 than to 7, perhaps ~5.


When can we make approximations?

We can decide this by comparing Ka to the initial concentration:

If then approximation will give less than 5% error, which is acceptable.

If then approximation is not acceptable and the calculation must be done using the quadratic formula.