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