A quantity of 6.0 g of a mixture of He and another gas occupies a volume of 2.0 L at 300 K and 740 mm Hg. The other gas freezes at 250 K. At 10 K, the pressure of the gas mixture is 6 mm Hg (at the sa

Question

A quantity of 6.0 g of a mixture of He and another gas occupies a volume of 2.0 L at 300 K and 740 mm Hg. The other gas freezes at 250 K. At 10 K, the pressure of the gas mixture is 6 mm Hg (at the same volume). What is the molecular mass of the gas?

TestHub · Accepted Answer

At 10 K only He is gaseous: n(He) = (6/760 × 2)/(0.0821 × 10) = 0.0192 mol, i.e., 0.077 g He. At 300 K: n(total) = (740/760 × 2)/(0.0821 × 300) = 0.0791 mol, so n(gas) = 0.0791 - 0.0192 = 0.0598 mol with mass 6.0 - 0.077 = 5.92 g. M = 5.92/0.0598 ≈ 99 g/mol. Confirms Ans: (a) 99.

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