K b = [ B H + ] [ O H ? ] [ B ] To find the pK a of X-281, you prepare a 0.093M

Question

K b =[BH + ][OH ? ] [B]

To find the pK a of X-281, you prepare a 0.093M test solution of X-281. The pH of the solution is determined to be 2.70. What is the pK a of X-281? To understand the relation between the strength of an acid or a base and its pK a and pK b values. The degree to which a weak acid dissociates in solution is given by its acid-ionization constant, K a . For the generic weak acid, HA , HA(aq)?A ? (aq)+H + (aq) and the acid-ionization constant is given by K a =[A ? ][H + ] [HA] Similarly, the degree to which a weak base reacts with H 2 O in solution is given by its base-ionization constant, K b . For the generic weak base, B , B(aq)+H 2 O(l)?BH + (aq)+OH ? (aq) and the base-ionization constant is given by K b =[BH + ][OH ? ] [B]

Explanation / Answer

Let HA represent X-281


.....HA...<=>...H+...+...A-

I...0.093.......0........0

C....-a.........+a.......+a

E...0.093-a......a........a


pH = -log[H+] = 2.70

a = [H+] = 10^(-pH) = 10^(-2.70) = 0.0019953 M


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

= a^2/(0.093 - a)

= 0.0019953^2/(0.093 - 0.0019953)

= 4.375 x 10^(-5)


pKa = -log Ka

= -log(4.375 x 10^(-5))

= 4.36

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