The balanced chemical equation for the combustion reaction is as given below:
C2Hx+(8+x4)O2→2CO2+x2H2O.
But 1 mole of C2Hx requires 3 moles of oxygen for complete combustion.
Hence, 3=8+x4
or, x=4.
The molecular weight of C2H4 is 28 g/mol.
Hence, 140 g of C2H4 corresponds to 14028=5 moles.