if a car produces 81g of {\rm CO_2}, how many grams of {\rm O_2} are used up in the reaction? the complete chem reaction is C2H6O+3O2--->2CO2+3H2O
I have no idea what C2H6O is but that doesn't matter.
Convert 81 g CO2 to mols. mols = grams/molar mass.
Using the coefficients in the balanced equation, convert mols CO2 to mols O2.
Finally, convert mols O2 to grams. g = mols x molar mass.