Finding Molar mass?

Question

A sample of gas of mass 2.74   occupies a volume of 639 mL at 25  degrees celsius and   1.00 atm pressure. What is the molar mass of the gas?

Answer 1

I'll assume the mass is 2.74 GRAMS and not some other unit of mass.

Use the ideal gas law to find the moles of gas ... PV = nRT

n = moles = PV/RT = (1.00 atm)(0.639 L) / (0.0821 Latm/Kmol)(298K)

n = 0.0261 moles

Since this is equal to 2.74 g, the molar mass = 2.74 g/0.0261 moles = 38.3 g/mole