A small tank of propane is opened and releases 30.0 L of gas at STP. What mass of gas was released?

Question

How do I go about to solve this kind of question?

Answer 1

Start with the molar mass of propane, which is 44.1 g/mol.

There are a few ways you could figure out the moles of propane. You could use the 22.4 L/mol conversion for STP of gases or plug everything into the ideal gas law with STP conditions.

30.0 L * 1mol/22.4L = 1.34 mol propane

OR

PV = nRT; T = 0C = 273K and P = 1 atm; R = 0.0821 Latm/molK

1atm*30.0L=n*0.0821Latm/molK*273K, solve for n = 1.34 mol propane

1.34 mol propane * molar mass gets you the mass. 1.34 mol * 44.1 g/mol = 59.1 g propane.

Hope that makes sense - I tend to try to find moles first because getting to grams is typically pretty easy from there (just molar mass conversion).

Answer 2

First let us determine the number of moles. 1 mole contains 22.4l of gas at stp. Yhen we find the molar mass of propane. Propane is C3H8 and has a molar mass of 44 g/mol. No we use dimensional analysis

30 x 1mol/22.4 L x 44 g / 1 mol = 58.9 g