Question

Calculate the concentration of all species in a 0.180 M solution of H2CO3 find : [H2CO3] , [HCO3^-] , [CO3^-2] , [H3O^+] , [OH-] Please explain steps, thank you.

Answer 1

There are no molecules of H2CO3 in a solution of "carbonic acid". Molecules of H2CO3 don't exist in aqueous solution. What we call carbonic acid is actually a solution of carbon dioxide in equilibrium with hydrogen ion and bicarbonate.

CO2(g) + H2O <==> H+ + HCO3- .............. Ka = 4.5 x 10^-7

HCO3^- is a weaker acid, dissociating to a slight extent to produce H+ and CO3^2-

HCO3^- <==> H+ + CO3^2- ....................... Ka = 4.7 x 10^-11

Ka = [H+] [HCO3-] / [CO2]
4.5x10^-7 = x&2up2 / 0.180
x = 2.85x10^-4

Ka2 = [H+] [CO3^^2-] / [HCO3-]
4.7x10^-11 = [CO3^2-

[CO2] = 0.180
[H+] = 2.85x10^-4 M
[HCO3^-] = 2.85x10^-4 M
[CO3^2-] = 4.7x10^11
[OH-] = 1.00x10^-14 / 2.85x10^-4 = 3.5x10^-11