Formula of this..?

Question

On complete combustion, 4.0g of a hydrocarbon give 11.0g of carbon dioxide and 9.0g of water.

What is the formula of the hydrocarbon?

(1) CH4
(2) C2H4
(3) C2H6
(4) C3H8

The answer is (1) CH4..why? I don't understand...please explain..thanks.

Answer 1

Molar mass of carbon dioxide: 44g/mol
11.0g of carbon dioxide ==> 0.25 mole of carbon dioxide ==> 0.25 mole of carbon atoms
Molar mass of water: 18g/mol
9.0g of water ==> 0.5 mole of water ==> 1.0 mole of hydrogen atoms.
Thus, hydrogen : carbon ratio is 4:1.
Only CH4 satisfy this requirement. Also do a senitery check:
The molar mass of CH4: 16 g/mol
4.0g of CH4 ==> 0.25 mole of CH4 ==> 0.25 mole of carbon atoms and 1 mole of hydrogen atoms.==> every thing fits well.
Understand??