Question

Calculate the percent dissociation of HF (Ka= 3.5x10^-4) in

(a) 0.050 M HF

(b) 0.50 M HF

Answer 1

Let a be the degree of dissociation of HF & c be its initial concentration

   HF          H⁺     + F^-       

initial conc    c 0    0

change    -ca    +ca    +ca

Equb conc    c(1-a )       ca ca

Dissociation constant of the acid HF ,

  

For weak acids the degree of dissociation(a) is very small, therefore 1-a may be taken as 1

                                                       

                                                        

(a) Given c = 0.050 M

             K_a = 3.5 x 10 ^-4

So

Percentage dissociation = 0.084 x 100 = 8.4 %

(b) Given c = 0.50 M

             K_a = 3.5 x 10 ^-4

So

Percentage dissociation = 0.026 x 100 = 2.64 %