# If a gas effuses #1.89# times as fast as #"COCl"_2(g)#, what is its molar mass?

##### 2 Answers

Graham's law states that :

For 2 gases this becomes:

So:

So:

The compound that effuses faster must have lighter molecules, if they are both at the same temperature.

Take the ratio that is

Square that value,

This is the *mass* ratio between heavier and lighter molecules, because these have the same mean kinetic energies and root mean squared speeds, as consequence of the **equipartition of energy** :

So the lighter molecules have mass

where 98.9 = C + O + 2Cl = 12.01 + 16.00 + 35.45ยท2 = 98.9.