Question

To find the pKa of X-281, you prepare a 0.092M test solution of X-281. The pH of the solution is determined to be 2.40. What is the pKa of X-281?

Answer 1

To find the pKa of X-281, you prepare a 0.092M test solution of X-281. The pH of the solution is determined to be 2.40. What is the pKa of X-281?

Lets assume X-281 is HA (Acid )

Ionization of HA

       HA(aq) + H2O (l)           ---------->H3O + (aq) + A- (aq)

By using ICE chart we find equilibrium concentration of each species

                HA(aq) + H2O (l)           ---------->H3O + (aq) + A- (aq)

I                0.092                                                   0                                    0

C            -x                                                         +x                   +x

E       ( 0.092-x)                                              x                          x

WE have been given pH.

We know pH = - log[H3O+] ; [H3O+] = Antilog ( -pH) = Antilog ( - 2.40 ) = 0.00398 M

[H3O+]= x = 0.00398 M = [A- (aq) ]   

We know ka for this reaction :

Ka = [H3O+] [A-]/ [HA]

= x²/ ( 0.092-x)

Lets plug value of x

Ka = (0.00398 )² / ( 0.092-0.00398 )

= 0.0001800

Pka = -log ka = -log 0.0001800 = 3.74

Pka of X-281 = 3.74