Question

A manometer attached to a flask contains $N{H}_3$ gas have no difference in mercury level initially as shown
in diagram. After the sparking into the flask, it have difference of 19 cm in mercury level in two columns.
Calculate % dissociation of ammonia.

• A. 50 %
• B. 25 %
• C. 75 %
• D. 100 %

Answer 1

The correct option is B 25 %

$\Delta P=h\rho g=\frac{19}{100}\times 13.6\times 9.8=25.32kPa$.
Convert the unit in atm
$\frac{25.32}{101.3}=0.25atm$
Total pressure $=1atm+0.25atm=1.25atm$
$2N{H}_3\rightleftharpoons 3H_2+N_2$

	$N{H}_3$	$H_2$	$N_2$
Initial pressure (atm)	1	0	0
Equilibrium pressure (atm)	1-2x	3x	x

$1-2x+3x+x=1.25$
$2x=0.25$
Hence, the degree of dissociation is $\frac{0.25}{1}\times 100=25$%