Solution 3.2

Example 3.2 Problem Statement:

Air is approximately 79 mole% N2 and 21 mole% O2. Calculate the average molecular weight of air by using the following formula:

    MWavg = yN2 * MWN2 + yO2 * MWO2


To solve Example 3.2:

The molecular weight of air can be calculated explicitly from the provided equation. The molecular weights of nitrogen and oxygen are known, so yN2 and yO2 are the only terms that must be calculated. Remember, yA is the mole fraction of A in the mixture, which is defined as:

    Equation 4:     yA = nA / ntotal

Since the given information is mole % and we need mole fractions, we can assume a basis to simplify our calculations. We will assume a basis of 100 moles total, which means that our sample of air contains 79 moles of N2 and 21 moles of O2. Now, each of the mole fractions can be calculated:

    For Nitrogen:
    yN2 = 79 moles of N2 / 100 moles total
    yN2 = 0.79
    For Oxygen:
    yO2 = 21 moles of O2 / 100 moles total
    yO2 = 0.21

Now, we have all of the terms we need to calculate the molecular weight of air.
    MWN2 = 28.0 g/mole
    yN2 = 0.79
    MWO2 = 32.0 g/mole
    yO2 = 0.21

    MWavg = 0.79 * 28.0 g/mole + 0.21 * 32.0 g/mole
    MWavg = 28.84 g/mole


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