V(CH3OH)= 2 litres
ρ(CH3OH) = 0,80 g/ml
v(O2) - ?
(How much O2 is it needed to burn with 2 litres of CH3OH?)
g CH3OH = volume x density = 2000 x 0.80 g/mL = ? grams.
CH3OH + 2O2 ==> CO2 + 2H2O
Here is a worked example. Just follow the steps.
http://www.jiskha.com/science/chemistry/stoichiometry.html