Untitled - Kelly Walsh High School

228 CHEMISTRY FOR THE UTTERLY CONFUSED
Don’t Forget!
Quick Tip
You should always write an equilibrium constant expression.
We can now create an ICE table:
HC 2H 3O 2 H C 2H 3O 2
Initial 0.300 0 0
Change x x x
Equilibrium 0.300 x x x
We can enter the values from the equilibrium line into the equilibrium constant
expression:
Ka 1.8 105 [H [x] [x]
[0.300 x]
][C2H3O2 ]
[HC2H3O2 ]
Your instructor may allow you to simplify this expression by assuming 0.300
x 0.300 M. If your instructor does not allow this simplification, you will need
to do a quadratic.
The hydrogen ion concentration is x [H ] 2.3 10 3 M. Thus, the solution
is acidic.
The pH of any acid solution must be below 7 and the [H ] must be greater than
10 7 M. The pH of any base solution must be above 7 and the [OH ] must be
greater the 10 7 M ([H ] < 10 7 M). If your results disagree with this, you have
made an error.
Now let us try an example using a weak base.
A 0.500 M solution of ammonia has a pH of 11.48. What is the K b of ammonia?