calculating mass/volume at STP

Question

1) What is the mass of 75.0 L of methane gas (CH₄) measured at STP?

2) What volume, in liters, will be occupied by 48.8 g of helium gas at STP?

3) What volume, in liters, at STP, is occupied by 3.30 X 10²³ molecules of oxygen gas?

4) What is the mass of 3.01 x 10²² molecules of carbon dioxide at STP?

Answer 1

These are all small gas molecules at STP (standard temperature and pressure), so we will assume ideal gas law (carbon dioxide is polar so would have the largest deviation from ideal gas behavior).

1 mol of ideal gas occupies 22.4 L at STP.

75.0 L CH4 * (1 mol / 22.4 L) = 3.348 mol CH4

molar mass of CH4 = 16.04 g / mol

3.348 mol CH4 * (16.04 g / mol) = 53.7 g CH4

We use 3 significant figures in the answer because 3 were given in the problem statement, "75.0"

Keep using 22.4 L / mol and each compounds molar mass to solve the rest of the problems. Also remember Avogadro's number, that there are 6.022*10^23 molecules per mole for the last 2 problems. If you need further assistance, please book a session with me!