Question

e Search this course Acid-Base Properties of Salts, and Buffers What is the pH of a 0.22M solution of sodium benzoate? K = 6.3 x 10" for benzoic acid, C,HCOOH. Would this solution be more or less acidic than a 0.22M solution of sodium acetate? K, = 1.8 x 10 for acetic acid, CH3 COOH. pH- This solution would be - acidic than a 0.22M solution of sodium acetate, Submit Answer 1 question attempt remaining

Answer 1

1)

use:

Kb = Kw/Ka

Kw is dissociation constant of water whose value is 1.0*10^-14 at 25 oC

Kb = (1.0*10^-14)/Ka

Kb = (1.0*10^-14)/6.3*10^-5

Kb = 1.587*10^-10

C6H5COO- dissociates as

C6H5COO- + H2O -----> C6H5COOH + OH-

0.22 0 0

0.22-x x x

Kb = [C6H5COOH][OH-]/[C6H5COO-]

Kb = x*x/(c-x)

Assuming x can be ignored as compared to c

So, above expression becomes

Kb = x*x/(c)

so, x = sqrt (Kb*c)

x = sqrt ((1.587*10^-10)*0.22) = 5.909*10^-6

since c is much greater than x, our assumption is correct

so, x = 5.909*10^-6 M

use:

pOH = -log [OH-]

= -log (5.909*10^-6)

= 5.2285

use:

PH = 14 - pOH

= 14 - 5.2285

= 8.7715

Answer: 8.77

2)

Ka of CH3COOH is less than Ka of C6H5COOH.

So, C6H5COOH is stronger acid.

Since stronger acid have weaker conjugate base,

CH3COO- would be stronger base than C6H5COO-.

So, pH of CH3COO- will be more.

Hence the solution is more acidic than sodium acetate

Answer: more