how many grams of methane are in a 2-L tank of methane at STP?

Question

This Is a practice problem for school 

Answer 1

When a problem states the conditions of STP, there is no absolute need to use the ideal gas law, although you certainly may do that.  But an easier method is to recall that 1 mole of any ideal gas at STP occupies 22.4 liters.  Memorize that.

Then, 2 L x 1 mole/22.4 L = 0.0893 moles of methane

0.0893 moles x 16 g/mole = 1.429 g = 1 g (to 1 significant figure)

Answer 2

Use the Ideal Gas Law, PV=nRT, but switch it around so n=PV/RT

 

P = 1 atm

V = 2 L

T = 273 K

 

n = (1 atm)(2 L)/((0.0821 L atm/mol K)(273 K))

n = 0.08923 mol

 

molar mass of methane = 16 g

 

(16 g/mol)(0.08923 mol) = 1.427 g

 

Rounding to significant digits gives 1 gram of methane.