Question

what is the ratio of average speed of He to HCl gas ?

Answer 1

Are you trying to find the root-mean-square speed?

Answer 2

yes .

Answer 3

Let's pick 310K as our temperature
\[\mu = (\frac{3k N T}{M})^{\frac{1}{2}}\]\[(\frac{3 R T}{M})^{\frac{1}{2}}\]\[\frac{(3(8.3145)(310))^\frac{1}{2}}{M^{\frac{1}{2}}}\]\[\mu = \frac{87.9345}{M^{1}{2}} m/s\]Now just stick the atomic weight for He and HCl in kg/mol into M

Answer 4

you can also use graham's law: \(\dfrac{rate_1}{rate_2}=\sqrt{\dfrac{M_2}{M_1}}\) ;and do it in one go.