The answers to Gravitational collapse and free fall time (spherical, pressure-free) describe the calculation of collapse time for a gas. For the case of neutral hydrogen gas with number density $10^4$/cm$^3$, how to find the collapse time, $t$. Also, how to find the collapse time for the case of 70% hydrogen and 30% helium gas (by mass).
I got $10^6$ years for the neutral hydrogen gas by using $d= (M*n)/N$, where $N$ is the Avogadro number, $n$ is the number density and $d$ is the mass density.
How do we do the calculation for the mixture of the two gases hydrogen and helium?