Question

2.15 g of an unknown gas at 71 degrees C and 1.10 atm is stored in a 1.75 L flask.
What is the density of the gas?
What is the molar mass of the gas?

Answer 1

Applying gas law:
PV=nRT
n/V= (1.10 atm) / ((0.08205746 L atm/K mol) x (71 + 273) K) = 0.0389 mol/L 
density: 
(1.10 g) / (1.75 L) = 0.628 g/L 
molar mass: 
(0.628 g/L) / (0.0389 mol/L) = 16.14 g/mol 
its molar mass is nearly equal to NH3 that is 17.