HELP! As quick possible.
Question : A hydrocarbon compound is burnt completely in air to form 7.2cm^3 of CO2 and 7.2 of water vapour. What is the molecular formula of the hydrocarbon compound? [ 24 dm^3 at room temp]

Question

HELP! As quick possible. 
Question : A hydrocarbon compound is burnt completely in air to form 7.2cm^3 of CO2 and 7.2 of water vapour. What is the molecular formula of the hydrocarbon compound? [ 24 dm^3 at room temp]

anonymous · Answer

Show working please

callisto · Answer

7.2 (unit?) of water vapour???

anonymous · Answer

7.2 cm^3

callisto · Answer

no. of mole of CO2 formed = (7.2/1000) / 24.0 = 0.0004
no. of mole of H2O formed = (7.2/1000)/24.0 = 0.0004

CxHy + (x+y/4)O2 -> xCO2 +(y/2) H2O
x = 0.0004
y/2 = 0.004 => y = 0.0008
=> CH2?

callisto · Answer

I think we can only get this but not the exact one, as there is insufficient information

anonymous · Answer

But the answer is C3H6

callisto · Answer

Is that all you've got for the question? No missing information like the molecular mass of the hydrocarbon?

anonymous · Answer

Nope. thats it. :(

mos1635 · Answer

Callisto is right
something is meesing
with these gata all you can get is CxH2x

anonymous · Answer

My teacher says try solve it using the ratio. But i dont understand how.

anonymous · Answer

it has to be an alkene (CnH2n) coz the ratio of the products are balanced...the ratio of products divided by 2 gives the number of moles of hydrocarbon used...but u need the mass of the hydrocarbon to get the molecular formula?

anonymous · Answer

eg :
if u take the complete combustion of C2H4
$$C_{2}H_{4} + O_{2} \rightarrow 2CO_{2} + 2H_{2}O$$
number of moles of products equal = means the hydro carbon belongs to the "alkene" group

Then u can find the no of moles of hydrocarbon used by divide the number of moles of Product divided by 2 =
In your case 0.0003= no of moles of product formed, then u get the no of moles of hydrocarbon used by dividing t with 2 = u get 0.00015

Now the point is u need to know the Mass of hydrocarbon to get the molecular formula 
$$n= \frac{mass}{molecular formula}$$

anonymous · Answer

I got it now. Sorry because its an objective question.
So, i guess we can pick according to the ratio. 
a. C2H5
b. C3H6
c. C3H8
d. C6H6 

thanks everybody. sorry because i didnt include the answer :/ sorry

anonymous · Answer

its B:)

mos1635 · Answer

yeap

callisto · Answer

only B is the compound that C:H is 1:2, as I've calculated before

anonymous · Answer

thts why it is B :P