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I have a mixture with 1 part of gas A, and 7 parts gas B. This ratio is in terms of mass.

Gas   Molar Mass (kg/mol)
A     0.17034
B     0.034

I'm trying to find the specific gas constant of the mixture.

$$ R_{\text{sp}} = \frac{R}{M} $$

I thought it might be $$ M = 0.17034\frac{1}{8} + 0.034\frac{7}{8} $$ but I can't work out if that makes sense given the units, or whether I should be using atomic mass or something.

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1 Answer 1

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The molar mass $(M)$ of a mixture with known mass fractions $(w_{i})$ is calculated from:

$$ \frac{1}{M} = \sum_{i}\frac{w_{i}}{M_{i}} $$

So in this case $$ \frac{1}{M} = \frac{1/8}{0.17034}+\frac{7/8}{0.034} = 26.469\ldots\\ M = \frac{1}{26.469\ldots} = 0.037779872\ \mathrm{kg}/\mathrm{mol} $$

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