Question

Caffeine (C_8H_10N_4O_2) is a weak base with a K_b value of 4 times 10^-4. The pH of a 0.01 M solution of caffeine is in the range of: a. 2-3 b. 5-6 c. 7-8 d. 9-10 e. 11-12

Answer 1

Answer:

The pH of the solution lies from 11 to 12.Hence, option e is correct.

Explanation:

The value of $K_b$ for caffine = $4\times 10^{-4}$

$CafOH(aq)\rightleftharpoons Caf(aq)+OH^-(aq)$

Initial

   0           0.01 M       0

AT equilibrium:

  x          (0.01 -x)M      x

$K_b=\frac{x(0.01-x)}{(x)}$

$4\times 10^{-4}=\frac{x(0.01-x)}{(x)}$

Solving for x:

x = 0.0096 M

The pOH of the solution is given by :

$pOH=-\log[OH^-}$

$pOH=-\log[x]$

$pOH=-\log[0.0096]$

pOH = 2.02

pH= 14 - pOH = 14 - 2.02 = 11.98

The pH of the solution lies from 11 to 12.