Specific gas constant for two gases
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
$$
\frac{1}{M} = \frac{1/8}{0.17034}+\frac{7/8}{0.034} = 26.469\ldots\\
M = \frac{1}{26.469\ldots} = 0.037779872\ \mathrm{kg}/\mathrm{mol}
$$