Question

if you combust 30kg of octane (about the amount in one tank for a small car), how many kilograms of CO2 will you emit?

Answer 1

Octane has a chemical formula of C8H18.

For complete combustion, the reaction is:

2C8H18 + 25O2 -> 16CO2 + 18H2O + heat

RMM(C8H18)=114
RMM(CO2) = 44

So 2*114 grams of Octane will produce 16*44 grams of CO2.

Can you take it from here?

Check my calculations.