Throughout this course, we encourage making approximations whenever possible. Our answers will be set up using a standard format. First, we write the equation. Then, we write the initial concentrations. Next, we write the change in concentrations. Finally, we write the equilibrium concentration, the initial plus the change.
For the weak acid HA,
HA H+ + A-
When c>>x, c-x is about the same as c:
When working problems, we usually allow this approximation and we often give numbers that lead to 'perfect' squares (4, 9, 16, 25).
One way to test an approximation is to make the approximation, solve for x, and then determine the ratio of x to c. Roughly speaking, this will give the fraction of the approximation. The approximate percent of approximation is:
Remember, with some extra effort you can calculate the answer without approximation.
What is the net effect of this? Well, most of these calculations are rarely better than 5 percent. So, for the most part, we're not throwing important information away.
