Here are the concentrations of the species present in out 0.010 M solution of acetic acid in water.

HOAc		H+   +	OAc-
0.0096 M		4.0x10-4 M	4.0x10-4 M

Let's use these values to calculate the equilibrium constant. First we will write the equilibrium expression. Then we will substitute the concentrations and calculate the value of the equilibrium constant K.