The comparison of pKa determination between carbonic acid and formic acid and its application to prediction of the hydration numbers

For formic acid, two water molecules can bring the calculated pKa into good agreement with the experimental data, so the hydration number of HCOOH and HCOO- is 2. For carbonic acid, the calculated pKa1 as three water molecules are added is good agreement with the experimental data, so the hydration number of H2CO3 and HCO3- is 3. The accuracy of pKa2 prediction can also be significantly improved as more water molecules are added. The best agreement is for n = 4 (the number of water molecules), so the hydration number of CO32− is 4.