What we are calling “Little x Problems” make up most of the equilibrium problems you will have to solve, from simple equilibrium to solubility to weak acids and bases. Little x Problems are those where the change between the current concentrations and those at equilibrium is very small. Let’s look at several examples to understand what they might mean.
Example 1
Let’s imagine that after setting up the ICE table we had the following equilibrium expression:
There are two ways to determine that “x” is small in this case Both start by considering the value of K. In this case, K is very small (10-14). At this point we can either think about the numerator or the denominator.
No matter which choice you make, you can determine that “x” must be small.
Example 2
After setting up the ICE table we have the following equilibrium expression:
Once again, there are two ways to determine that “x” is small in this case Both start by considering the value of K. In this case, K is very large (1023). At this point we can either think about the numerator or the denominator.
NOTICE!
"Little x Problems" can occur with large or small K values. You have to think it through each time!
X is only little with either big or little values of K. mid-range K’s NEVER produce little x problems.
Now, let's solve a little x problem.
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