a gas occupies 2.7 liters and weighs 1.72 grams at STP. What is the molecular weight?

Question

a) 107 g/mol
b)864 g/mol
c)38.6 g/mol
d)14.3 g/mol

Answer 1

At STP, an ideal gas has a volume of 22.4 Liters per mole.
We will assume that the gas we are discussing is an ideal gas.

We are told that the volume of the gas is 2.7 Liters at STP, so from this we can calculate the number of moles which are present in the gas.

Moles = 2.7 Liters / 22.4 Liters per mole
Moles = .1205 moles

We are told that the gas has a mass of 1.72 grams. We just found that the gas sample contains .1205 moles of this gas. From these two pieces of information we can calculate the molar mass of the gas.

Molar mass = mass / moles
Molar mass = 1.72 grams / .1205 moles
Molar mass = 14.27 grams per mole

14.27 grams per mole (the answer I just calculated) is pretty close (it can be rounded up to one decimal place) to choice d), 14.3 grams per mole.
So the answer is choice d).