At STP, 0.160 L of a gas weighs 0.400 g. Calculate...?

Question

At STP, 0.160 L of a gas weighs 0.400 g. Calculate the molar mass of the gas. (g/mol)

any help is appreciated

Answer 1

For these problems, you use

PV = nRT

This tells you the number of moles. Divide the number of grams (in your case, 0.400 g) by a number of moles, and that is your answer.

There is another way which some people prefer. At STP, 1 mol of ideal gas occupies 22.4 L. So you have 0.160/22.4 moles.

I prefer the first way, because it works under any conditions, while the second way only works at STP.