Question

What is the Ph of na2co3 0.05m and 0.1 m solution?

Answer 1

Na2 CO3 in water => 2 Na+ + CO3^2-      --- (1) 
    CO3^2-  + H2O  ===>  HCO3- + OH-
    HCO3- + Na +   ===> NaHCO3

So the remaining OH- ions contribute for the basicity of the solution.  We have a NaOH solution which is basic and completely dissociating in water.

Let   M = Molarity of Na2CO3 solution.

Dissociation constant Kb of  Na2 CO3 = 0.0002128  for  eq (1)
So       [OH-] = [HCO3-] =  [CO3^2-]  = Kb * M
       pOH = - Log [OH-] = - Log 0.0002128 - Log M
                  = 3.672 - Log M
       pH = 14 - pOH = 10.328 - Log M
            = 10.328 - Log 0.05 = 11.63   for 0.05 Molar soln
            = 10.328 - Log 0.1  = 11.328   for  0.1 Molar soln