Weak Acids
Note that polyprotic acids have more than one value and each successive value decreases, often by a factor of about 10-4 or 10-5.
H2CO3 + H2O ? H3O+ + HCO3- Ka = 4.45 x 10-7
HCO3- + H2O ? H3O+ + CO32- Ka = 4.7 x 10-11
The larger the value of Ka, the more ionization occurs in solution. For a given initial concentration, a larger Ka gives a larger [H3O+] and a lower pH.