How do I calculate the number of moles in 11.2 L of CO2 at STP?

At STP, one mole of any gas occupies 22.4 litres (this is the molar volume). So for volume problems, you just divide the given volume by 22.4. Here, moles of CO2 = volume given divided by molar volume = 11.2 / 22.4 = 0.5 mole. You only use mass and molar mass (44 g/mol for CO2) when the question gives you grams instead of litres. A quick check: 0.5 mole of CO2 also weighs 0.5 times 44 = 22 g, and contains 0.5 times 6.022 times 10^23 = 3.011 times 10^23 molecules. So mole is the central link between volume, mass, and number of particles. Pick whichever bridge the question hands you.