Question

When 100 grams of He and 100 grams Ne are placed in a container, the partial pressure of Ne is found to be 3 atm. What is the partial pressure of He?

(1) 0.6 atm

(2) 3 atm

(3) 6 atm

(4) 15 atm

(5) 18 atm

Answer 1

Number of moles of Ne is , n' = mass/molar mass

                                           = 100 g / 20 (g/mol)

                                            = 5 mol

Number of moles of He , n'' = mass/molar mass

                                       = 100 g / 4(g/mol)

                                      = 24 mol

So total number of moles , n = n'+n" = 5+24 = 29 mol

Mole fraction of Ne , X' = n' / n

                                 = 5 / 29

                                 = 0.172

Mole fraction of He , X" = n" / n

                                  = 25 / 29

                                 = 0.862

We know that partial pressure , p = Total pressure x mole fraction

So total pressure , P = partial pressure of Ne / mole fraction of Ne

                              = 3 atm / 0.172

                              = 17.4 atm

Partial pressure of He = Total pressure x mole fraction of He

                               = 17.4 atm x 0.862

                               = 15 atm

Therefore option (4) is correct