gs The composition of a gas mixture is 20% O2, 20% N2 and 60% He by mass analysis. What is the molar mass of this mixture

Explanation:

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In this case, since the molar mass of a gas mixture can be computed as a weighted average containing mole fractions:

Now, since we are given the by mass percent of each gas, by assuming 1 g of mixture, we therefore have 0.20 g of oxygen, 0.2 g of nitrogen and 0.6 g of helium; next we compute the moles of each gas:

Next, we compute the mole fractions:

Now, we compute the molar mass of the mixture as follows:

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the answer to your question is c