I haven't checked the math, but carbon from the fuel binds with oxygen from the air. CO2 consists of one carbon atom (12g/mol), two oxygen atoms (16g/mol). So,

I haven't checked the math, but carbon from the fuel binds with oxygen from the air. CO2 consists of one carbon atom (12g/mol), two oxygen atoms (16g/mol). So, most of the weight comes from the oxygen from the air, rather than the fuel itself.