Question

Predict whether a solution containing the salt KHCO₃ will be acidic, basic, or neutral. Write the appropriate equilibrium equation(s) and support your prediction with an appropriate statement and calculation. The step-wise dissociation constants for carbonic acid are: K_a1= 4.2x10^-7 & K_a2= 4.8 x 10^-11

Answer 1

Show ionization of salt

KHCO3(aq) --- > K+(aq) + HCO3^- (aq)

We get HCO3^-

Lets write the reaction of this base with water

HCO3^- + H2O (l) ---> CO^2- + H3O+     …… 1

And

HCO3^- + H2O (l) --- >H2CO3   + OH-     …….2

We got two reactions, the reaction that has more equilibrium constant, will be faster.

Write reaction of acid for ka1

H2CO3(aq) + H2O ---- > HCO3-(aq) +H3O+(aq)   …..3

For ka2

HCO3-(aq)+ H2O(l) ----> CO3^2- (aq) + H3O+(aq)   …..4

Reaction 1 is ka2 reaction. (Ka2 = 4.8 x 10-11)

Reaction 2 is kb2 reaction. ( reverse to ka1)

Lets calculate kb2

Kb2 =1 .0 x 10^-14 / ka1 = 1.0 E-14 / 4.2 x 10^-7 = 2.38 x 10^-8

Now we compare kb2 and ka2 , kb2 is more than ka2 therefore reaction 2 is faster than reaction 1 and therefore, KHCO3 is basic salt.