At STP, 0.280 L of a gas weighs 0.400 g. Calculate the molar mass of the gas.
3 Answers

molar mass = g / mole right?. you know grams, if you could find moles you could calculate molar mass…
assume the gas is ideal…
PV = nRT
n = moles = mass / mw
PV = (mass / mw) RT
mw = mass x RT / PV
= 0.400 g x (.0821 Latm/moleK) x (273K) / (1 atm x 0.280 L)
= 32.g /mole
*********** alternately ***********
you could remember that 1 mole of an ideal gas has a volume of 22.41 L at STP from here….
V = nRT/P = 1 mole x .0821 Latm/moleK x 273 K/ 1 atm = 22.41 L
so 0.280 L x 1 mole / 22.41 L = 0.0125 mole
finally
molar mass = 0.400g / 0.0125 mole = 32.0 g/mole
your choice…

1 mole of gas at STP occupies 22.4L.
0.280 L / 22.4 L = 0.0125 moles of gas.
0.400g / 0.0125 moles = 32g/mol. Molar mass of the gas.

At STP 1 mol of an ideal gas occupies 22.4 L
0.280 L / 22.4 L = 0.0125 mol
0.400 g / 0.0125 mol = 32.0 g/mol (molar mass = 32.0)