0732. Solving a chemistry problem

5. Three identical vessels (of equal volume and equal mass) contain gases under the same conditions. The first vessel, filled with hydrogen, has a mass of 24.80 g. The mass of the second, filled with oxygen, is 36.80 g. The third vessel contains a mixture of 40% (by volume) nitrogen and an unknown gas and has a mass of 29.44 g. Determine the unknown gas .