# Past paper chemistry question is confusing me. Help

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Thread starter 5 years ago
#1
Complete combustion of 50 cm3 of a hydrocarbon vapour gave 350 cm3 of carbon dioxide?

both gas volumes being measured at the same temperature and pressure. The
formula of the hydrocarbon could be:

A: C8H18

B: C7H16

C: C6H14

D: C5H12

Could some one explain the answer?

I went with my gut instinct and divided 350 by 50. This gave me 7 so I went with B, the answer is correct but I do not understand why???? Has this got something to do with Avogadro??

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5 years ago
#2
(Original post by TheonlyMrsHolmes)
Complete combustion of 50 cm3 of a hydrocarbon vapour gave 350 cm3 of carbon dioxide?

both gas volumes being measured at the same temperature and pressure. The
formula of the hydrocarbon could be:

A: C8H18

B: C7H16

C: C6H14

D: C5H12

Could some one explain the answer?

I went with my gut instinct and divided 350 by 50. This gave me 7 so I went with B, the answer is correct but I do not understand why???? Has this got something to do with Avogadro??

Yes, this is Avogadro's law, which says that the volume of every gas contain the same number of moles.

From this you get Gay Lussac's law of combining volumes, which says that you can treat the coefficients of a balanced equation as gas volumes as well as moles.

Hence as:

CxHy + nO2 ---> xCO2 + y/2H2O

Then 1 volume of hydrocarbon CxHy produces x volumes of carbon dioxide.

So if 50cm3 CxHy makes 350 cm3 of CO2, then x = 350/50 = 7
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Thread starter 5 years ago
#3
(Original post by charco)
Yes, this is Avogadro's law, which says that the volume of every gas contain the same number of moles.

From this you get Gay Lussac's law of combining volumes, which says that you can treat the coefficients of a balanced equation as gas volumes as well as moles.

Hence as:

CxHy + nO2 ---> xCO2 + y/2H2O

Then 1 volume of hydrocarbon CxHy produces x volumes of carbon dioxide.

So if 50cm3 CxHy makes 350 cm3 of CO2, then x = 350/50 = 7
oh I see, thank you so much. Oh it's very simple actually
0
5 years ago
#4
(Original post by TheonlyMrsHolmes)
oh I see, thank you so much. Oh it's very simple actually
Chemistry is simple - it's all common sense ...
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Thread starter 5 years ago
#5
(Original post by charco)
Chemistry is simple - it's all common sense ...
I would have to say yes and no
0
5 years ago
#6
(Original post by charco)
Chemistry is simple - it's all common sense ...
I wouldn't say the average person would be able to complete an A2 level chemistry paper. Or AS for that matter.
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