Problem in As Chemistry

[elbeyon]

Since my visit to this forum I have seen dynamic and helpful minds ! You people solved many of my problems and I am grateful to that !
This time I am having problem regarding this Question ! Please do help !

When 10 cm^3 of the hydrocarbon is burned in 70cm^3 of oxygen (an excess),the final gaseous mixture contains 30cm^3 of carbon dioxide and 20cm^3 of unreacted oxygen. All gaseous volumes were measured under identical conditions.
What is the formula of the hydrocarbon?



Please solve it !

 

[destined007]

Number of moles is proportional to volume.
Therefore, lets assume that:
10 cm^3 is 1 mole
30 cm^3 is 3 moles
70 cm^3 is 7 moles and so on.

equation of combustion will be:
CxHy + O2 ---------> CO2 + H2O + O2
1 mole + 7 moles --> 3 moles + z + 2 moles

First, find x
The number of moles of carbon dioxide produced is the product of the number of moles of hydrocarbon and the number of carbon atoms it contains.
1 * x = 3
x=3

Now. find y
Initially, from 7 moles of O2, 3 moles is removed to produce CO2, and 2 moles remain unreacted. So the remaining 2 moles (7-(3+2)) must have reacted with hydrogen to produce water. There is only one oxygen atom is water so the moles of water produced must be 4 moles. Use the same procedure as for carbon.
The number of moles of water produced is the product of the number of moles of hydrocarbon and the number of hydrogens it contains divided by 2 (as there are two hydrogens in water).
1 * y/2 = 4
y=8.

formula is:
C3H8