[H+][C2H3O2-]
Ka = ------------
[HC2H3O2]
Substitute the given information, and solve for the unknown quantity.
[H+] [C2H3O2]
1.75 X 10--5 mol/L = ----------------
0.100 mol/L
NOTE:
Although the actual concentration of the acetic acid is less than 0.100 mol/L
after ionization, it is a negligible amount (less than 5 your future course work,
the amount of ionized acid or base in this type of problem should be
considered to be a negligible amount.
Substitute the variable x for H+ and C2H3O2- since for every dissociated molecule
of HC2H3O2, one hydrogen ion and one acetate radical are produced.
x2 = (1.75 X 10-5 mol/L) (0.100 mol/L) = 1.75 X 10-6 mol2/L2
+
x = [H ] = 1.32 X 10-3 mol/L
NOTE:
By convention, the equilibrium constants of weak electrolytes are written
without units. However, they do have units.
Section III. DISSOCIATION OF WATER
9-8.
DISCUSSION
Although water is not an electrolyte, its molecules do have a limited tendency to
+
-
dissociate into H and OH ions.
+
-
HOH <====> H + OH
It can be shown that the tendency of water to dissociate is given by:
-
[H+] [OH ]
K = ----------
[HOH]
At 25C, the value of K is 1.8 X 10-16.
MD0837
9-7