Solid NH4N3 decomposes explosively according to the following equation:

Question

NH4N3(s) ⇌ 2N2(g) + 2H2(g)

A small amount of ammonium azide was placed in a sealed container and the air was removed. The sample was then detonated. At equilibrium the total pressure in the container was found to be 421,959.129 Pa. Calculate Kp.

Answer 1

Assuming the temperature was at STP (0°C).

Consider a 1-L volume.
Total pressure
= 421959.129Pa
= 421.96 kPa
= 4.1654 atm at 0°C
Partial pressure of each product
= 2.08272 atm

This implies
P_N2=P_H2
=2.08272 atm

Kp=P_N2²P_H2²
= 2.0827^4
= 18.8158475075608
= 1.882*10¹ approx.

Note:
solids do not enter into the definition of equilibrium constants.
In any case, in an explosion, we assume all (solid) reactants have been consumed.