CBSE Class 12-science Answered

?T_f  = K_f x m

      = 1.86 x 0.50 = 0.93 K

Van't Hoff factor (i) = 1.0/0.93  =1.0753

Since fluoroacetic acid undergo dissociation as 

CH₂FCOOH  ---> CH₂FCOO^-  + H⁺

So, ?  = (i -1) / (n-1)  where ? is the degree of dissociation

Here n = 2

?= 1.0753 - 1 =0.0753

Now  

K_a = [ CH₂FCOO^-  ] [ H⁺] / CH₂FCOOH 

where,

[ CH₂FCOO^-  ] =  0.50 x 0.0753 =0.03765

[ H⁺] = 0.50 x 0.0753 =0.03765

[CH₂FCOOH] = 0.50 (1 - 0.0753) = 0.462

Thus, K_a = 0.03765 x 0.03765/0.462

             = 3.07 x 10^-3