A mixture of one mole each of H₂, He and O₂ each are enclosed in a cylinder of volume V at temperature T. If the partial pressure of H₂ is 2 atm, the total pressure of the gases in the cylinder is:

Since H₂, He and O₂ are all non-reactive gases, we can use Dalton's Law of Partial Pressure to calculate the total pressure of gases by Dalton's Law of Partial Pressure in a mixture of 'i' non-reactive gases,  $P_i = P_{T }\times x_i$

where P_i is pressure of gas i in the mixture of gases; x_i is the mole fraction of gas i in the mixture

P_T is the total pressure exerted by those gases

$P_{H_2 }= 2 atm$

So, $P_T = \frac{P_{H_2}}{X_{H_2}} = \frac{P_{H_2}}{{\displaystyle \frac{n_{H_2}}{n_{H_2}+n_{O_2}+n_{He}}}}$

$= \frac{2}{{\displaystyle \frac{1}{1+1+1}}} = 2\times 3 = 6$

Hence, correct option is: (C)  P_T = 6 atm