(c)
$N_{2}\ + \ \ \ 3H_{2} \rightleftharpoons \ \ \ \ 2{NH}_{3}$
1 3 0 initially
$1 - 0.81\ \ \ 3 - 2.43$
$\left( \mathbf{=}0.19 \right)\left( \mathbf{=}0.57 \right)\ \ 1.62$ at equilibrium
Number of moles of $N_{2} = \frac{28}{28} = 1\ mol$
Number of moles of $H_{2} = \frac{6}{2} = 3\ mol$
Number of moles of $NH_{3} = \frac{27.54}{17} = 1.62\ mol$
$\mathbf{\therefore\ }K_{c} = \frac{\left\lbrack NH_{3} \right\rbrack^{2}}{\left\lbrack N_{2} \right\rbrack\left\lbrack H_{2} \right\rbrack^{3}} = \frac{\lbrack 1.62\rbrack^{2}}{\lbrack 0.19\rbrack\lbrack 0.57\rbrack^{3}} = 75$