Question

) PABA refers to para-aminobenzoic acid which is used in some sunscreen formulations. If a 0.055 M solution of PABA has a pH of 2.96, determine the Ka of PABA. What kind of problem do you think thisis

Answer 1

Answer: $K_a$ of PABA is 0.000022

Explanation:

$NH_2C_6H_5COOH\rightarrow H^++NH_2C_6H_5COO^-$

  cM              0             0

$c-c\alpha$        $c\alpha$          $c\alpha$

So dissociation constant will be:

$K_a=\frac{(c\alpha)^{2}}{c-c\alpha}$

Give c= 0.055 M and $\alpha$ = ?

$K_a=?$

$pH=-log[H^+]$

$2.96=-log[H^+]$

$[H^+]=1.09\times 10^{-3}$

$[H^+]=c\times \alpha$

$1.09\times 10^{-3}=0.055\times \alpha$

$\alpha=0.02$

Putting in the values we get:

$K_a=\frac{(0.055\times 0.02)^2}{(0.055-0.055\times 0.02)}$

$K_a=0.000022$

Thus $K_a$ of PABA is 0.000022