If 84 g of CO2 were produced, how many grams of butanol were burned?

Answer 1

C4H9OH + 6O2 ==> 4CO2 + 5H2O

mols CO2 = grams CO2/molar mass CO2.
Use the coefficients to convert mols CO2 to mols butanol.
Now convert mols butanol to grams butanol.
g = mols butanol x molar mass butanol.