Calculate the concentration of all species in a 0.165 m solution of h2co3.

(1) H2CO3 + H2O --> HCO3- + H3O+ [Ka = 4.5E-7]  

(2) HCO3- + H2O --> CO32- + H3O+ [Ka = 4.7E-11]  

Observe that the two Ka values differ from each other drastically. This will mean that in terms of pH, ONLY THE FIRST IONIZATION WILL AFFECT pH.  

(1) H2CO3 + H2O --> HCO3- + H3O+ [Ka = 4.5E-7]  

I0.165M0  

C-x+x  

E0.165-xx  

4.5E-7 = x^2/(0.165-x) <-- use the appoximation rule because Ka is very small  

2.72E-4 = x  

Thus, from equation (1), we see that the [H3O+] = 2.72E-4M, [H2CO3] = 0.165M, and [HCO3-]= 2.72E-4M. Note that THIS step will determine the pH of the polyprotic acid  

To calculate the last species, just transfer these values the next equation  

(2) HCO3- + H2O --> CO32- + H3O+ [Ka = 4.7E-11]  

I2.72E-40  

C  

E  

4.7E-11 = (x*2.72E-4)/2.72E-4  

4.7E-11 = x = [CO32-]  

To calculate the OH- from H3O+, just find the pH of H3O+, subtract it from 14 to find the pOH, and find the [OH-] from that.  

every chemical equation adheres to the law of conservation of mass, which states that matter cannot be created or destroyed. therefore, there must be the same number of atoms of each element on each side of a chemical equation.