chem一條

The combustibility of hydrogen is much higher than that of carbon. Therefore, a hydrocarbon with a higher carboncontent would undergo incomplete combustion more readily.

Each molecule of ethyne (C2H2) contains 2 carbon atomsand 2 hydrogen atoms.
They mole fraction of carbon in ethyne = 2/(2 + 2) = 1/2

Each molecule of ethene (C2H4) contains 2 carbon atomsand 4 hydrogen atoms.
The mole fraction of carbon in ethene = 2/(2 + 4) = 1/3

Ethyne has a higher carbon content than ethene. Hence, ethyne would undergo incomplete combustionmore readily than ethene. In other words,ethene would undergo complete combustion more readily than ethyne.