A 250 ml cylinder contains krypton gas at 500mm pressure. Another cylinder of 500 ml contains helium gas at 950 mm pressure.

Calculate the pressure if the two gases are allowed to mix together through a stopcock.(temp is constant).

Here we will first calculate the partial pressures of the two gases after they are mixed together and then add them up. Final volume = ( 250 + 500 ) = 750 ml

For partial pressure of Krypton, we have 

                                               P₁V₁ = P₂V₂                                            (1)

where P₁ = 500 mm of Hg, V₁ = 250 ml, P₂ = ? and V₂ = 750 ml

Substituting the above values in equation (1), we get

                                  P₂ = 166.667 mm of Hg

For partial pressure of He, we have 

                                                       P'₁V'₁ = P'₂V'₂                                       (2)

Here P'₁ = 950 mm of Hg, V'₁ = 500 ml, P'₂ = ? and V'₂ = 750 ml

Substituting the above values in equation (2), we get

                                P'₂ = 633.33 m of Hg

Thus the total pressure of the system is 

                                     P_total = P₂ +  P'₂

                                               = 166.667 + 633.333

                                               = 800 mm of Hg