Finding molecular weight from specific heat at constant volume

[TheTourist]

The specific heat at constant volume of an unknown solid is measured at high temperature to be 319.5Jkg^-1K^-1. Estimate its molecular weight.



Cv=3R




Attempt.
I used the above equation to get a mass of the solid to be 0.0128kg but I have no idea of what to do next!

 

[Mapes]

I used the above equation to get a mass of the solid to be 0.0128kg but I have no idea of what to do next!

How did you calculate this? (Please include your units.)