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A chemistry question watch

1. I would very much appreciate it if anyone can help me with his question I'm stuck on. Thank you very much!

A technique called eudiometry can be used to learn about benzene. It relies on the fact that at a give temperature and pressure one mole of any gas has the same volume. If 10cm3 benzene vapour is burned with 100cm3 of oygen, the volumes of gases at the end are O2 = 25cm3, CO2 = 60cm3, steam= 30cm3. Why is the molecular formula for benzene C6H6?
2. (Original post by Mimo)
I would very much appreciate it if anyone can help me with his question I'm stuck on. Thank you very much!

A technique called eudiometry can be used to learn about benzene. It relies on the fact that at a give temperature and pressure one mole of any gas has the same volume. If 10cm3 benzene vapour is burned with 100cm3 of oygen, the volumes of gases at the end are O2 = 25cm3, CO2 = 60cm3, steam= 30cm3. Why is the molecular formula for benzene C6H6?
General equation for the combustion of a Hydrocarbon is,
CxHy + (x +y/4)O2 ----> xCO2 + (y/2)H2O

Then if you fill in the volumes,

CxHy + (x +y/4)O2 ----> xCO2 + (y/2)H2O
10 75 (100-25) 60 30

work out ratio,
CxHy + (x +y/4)O2 ----> xCO2 + (y/2)H2O
1 7.5 6 3

Therefore using balancing numbers,

From xCO2 -----> x = 6
From (y/2)H2O ----> y = 6

therefore compound must be C6H6.

If I remember correctly this is also called Gay Lussac's Law
3. Thanks so much, Hawk. That was very useful... although it took me a while to fully understand it.

Mimo
4. (Original post by Mimo)
One mole of gas occupied 24 dm3 regardless of the gas.
10 cm3 of benzene + 100 cm3 of oxygen conbusted
produces are O2 = 25cm3, CO2 = 60cm3, H2O= 30cm3
ok consider the equation
CxHy + ?O2 --> ?CO2 + ?H2O... as we can see... all carbon in CO2 originate from benzene and all hydrogen in H2O also originate from benzene...
So... C : H = 60 : (2X30)... note TWO hydrogen in H2O
so... simplest C : H ratio is 1:1... thus x=y

so rewrite equation... CxHx+ ?O2 --> xCO2 + (x/2)H2O...
so calculate the oxygens... 2x+x/2 on right... so 5x/2 on left... but notice that it is O2... so we divide by two to get 5x/4 O2...
CxHx+ (5x/4)O2 --> xCO2 + (x/2)H2O

now as the mole ratios...
10 CxHx: 75 O2(100 orignal - 25 remaining): 60CO2: 30 H2O
so... 5x/4 = 75, x=60, x/2=30 --> x=60 for all cases....

so does C60H60+ 60O2 --> 60CO2+ 30H2O look right? no.... cause remeber that its 10 CxHx not 1 CxHx in the moles ratio so you divide x by 10...

thus true x=6... C6H6+ 6O2 --> 6CO2+ 3H2O
5. Even better. Thanks.

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Updated: February 18, 2005
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