What is the composition of car-exhaust gas? Is it solely carbon dioxide?
Smoke and exhaust fumes would also contain water vapour and carbon dioxide, all of which possess mass, and also some incompletely combusted hydrocarbon. Of course you could assume that the mass represented only carbon, but this is physically unrealistic.
It is not solely carbon dioxide.
Exhaust gas comes from the combustion of a mixture of hydrocarbons (gasoline/petrol) with air.
#underbrace("N"_2 + "O"_2)_color(red)("air") + "C"_text(x)"H"_text(y) → "N"_2 + "H"_2"O" + "CO"_2#
Thus, most of the exhaust gas consists of nitrogen, water vapour, and carbon dioxide.
A relatively small part of the exhaust gas consists of undesirable products.
Some of the hydrocarbons are not completely oxidized, so minor constituents include carbon dioxide and particles of soot.
The high temperatures inside the engine cause some of the nitrogen to form various oxides of nitrogen (
#"N"_2"O, NO, NO"_2#, etc.). Chemists generally give them the general formula #"NO"_text(x)#.