Let the partial pressure of NItrogen gas be x cm of Hg
Reaction: 2NH3(g)⇌N2(g)+3H2
Before sparkling
pressure: 76 0 0
After sparkling, when equilibrium is reached,
Pressure: 76-2x x 3x
So, final pressure = 76−2x+x+3x = 76+2x
Given, Final pressure = 82 cm of Hg
Equating, 82 = 76+2x
We get, x = 3 cm of Hg
Partial pressure of Hydrogen = 9 cm of Hg