Dennis1590

They are doing a bad job in explaining how the pH changes when adding carbonates to water. Either sodium bicarbonate or another carbonate. I tried to understand it by myself and google searches but still haven't found a way. This is the problem: So the water has a pH of 6.35. Which is the pKa value of H2CO3/HCO3-. So in solution at this pH there should be 50% H2CO3, 50% HCO3- and 0% CO32-. So in general they now start calculating stuff with the electroneutrality equation which is: H+= HCO3- + 2*CO32- + OH- . Lets assume that the total of carbonates species is 0.03 mol/L at a pH of 6.35. Than 50%*0,03 = 0.015 mol/L of H2CO3 and 0.015 mol/L of HCO3- in solution. There is also kw/10-6.35=10-7.65 mol/L OH- present in solution. So filling this in the electroneutrality equation: H+= 0.015 + 10-7.65 = 0.015 mol/L. So this is a bit confusing. Since pH is 6,35 it should be H+= 10-6.35 mol/L. Not 0.015 mol/L. My question is. What am I missing here. Can somebody help clear the confusion.