Explanation

(b)
$NH_{2}COONH_{4}(s) \rightleftharpoons 2{NH}_{3}(g) + {CO}_{2}(g)$
1 2 4
If $p$ is the total pressure at equilibrium
$K_{p} = \frac{\left\lbrack NH_{3} \right\rbrack^{2}\lbrack CO_{2}\rbrack}{\lbrack NH_{2}COONH_{4}\rbrack}$
$2.9 \times 10^{- 5} = \frac{\left( \frac{2p}{3} \right)^{2} \times \frac{p}{3}}{1}$
$2.9 \times 10^{- 5} = \frac{4 \times p^{3}}{27}$
$\Rightarrow p^{3} = \frac{27 \times 2.9 \times 10^{- 5}}{4}$
$= 1.9575$
$p = \sqrt[3]{1.9575} = 0.058$