1 mole of H2 gas is contained in a box of volume V = 1.00 m³ at T₁ = 300 K and the gas is heated to a temperature of T₂ = 3000 K and the gas gets converted into a gas of hydrogen atoms. The final pressure would be (considering all gases to be ideal) :

20 times the pressure initially

When the molecules break into atoms, the number of moles would become twice.

Now, by ideal gas equation : 

P = pressure of gas, n = number of moles

R = gas constant, T = temperature

We have pV = nRT

As volume (V) of the container is constant.

As gases break, number of moles becomes twice of initial number, so n₂ = 2 n₁

So, \(p \propto nT\)

\(\Rightarrow \frac{p_2}{p_1} = \frac{n_2 T_2}{n_1 T_1} = \frac{2n_1*3000}{n_1* 3000} = 20\)

\(\Rightarrow p_2 = 20 p_1\)

Hence, final pressure of the gas would be 20times the pressure initially.