It is found that an unknown gas has a density of 2.5 times that of Oxygen gas of the same Temperature and Pressure. What is its molecular weight?

Answer 1

It will be 2.5*32 = ?
The long way is to calculate the density of O2 (32g/22.4L = 1.43 g/L), multiply by 2.5 for a density of the unknown of 3.57 g/L, so 22.4L has a mass of 22.4*3.57 = 80 which is a mole.