HELP PLEASE!! Molar mass of unknown gas Hey, I would really appreciate a little help. I'm kind of confused :( with this question: A sample bulb contained 0.712 g of H2 (g) at a certain temperature and pressure. Under the same conditions of temperature an pressure, the bulb can hold 13 g of an unknown gas. What is the molar mass of the unknown mass?? Does anybody know if I have to apply the ideal gal law and how?? Thank you!!