Question

10cm³ of a gaseous hydrocarbon, C_xH_y required 30cm³ of oxygen for complete combustion. If steam and 20cm³ of Carbon (IV) oxide were produced, what is the value of x in C_xH_y. 

Answer 1

C_xH_y(g)+O_2(g)→CO_2(g)+H₂O_(g) 
10cm³    30cm³       20cm³
Their volumes to mole ratio
C_xH_y(g)+O_2(g)→CO_2(g)+H₂O_(g) 
1mole    3moles      2moles
C_xH_y(g)+3O_2(g)→2CO_2(g)+H₂O_(g) 
Then 2 atoms of carbon were present in C_xH_y and 4 atoms of Hydrogen.
C₂H_4(g)+3O_2(g)→2CO_2(g)+2H₂O_(g)
Thus x = 2