Empirical formula from Combustion values Watch

Akib Raihan
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10cm3 of a gaseous hydrocarbon was mixed with excess oxygen and ignited. The gas volumes were measured at room temperature and pressure before and after combustion and it was found that the total gas volume had contracted by 20 cm3 . Given that combustion was complete . so How can I find the Formula ? please teach me from the basic . I need to learn as soon as possible
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username3718068
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(Original post by Akib Raihan)
10cm3 of a gaseous hydrocarbon was mixed with excess oxygen and ignited. The gas volumes were measured at room temperature and pressure before and after combustion and it was found that the total gas volume had contracted by 20 cm3 . Given that combustion was complete . so How can I find the Formula ? please teach me from the basic . I need to learn as soon as possible


pV = nRT

we know that n is directly proprtional to V because T and P are directly proportional.

now refer to equation:

CxHy + (2x + y/2) O2 => xCO2 + y/2 H2O (couldnt be bothered for state symbols)

because there are 10cm3 of CxHy , there are :
10(2x + y/2) = 20x + 5y cm3 of oxygen being used up
10x cm3 of carbon dioxide
5y cm3 of water vapour

the overall decrease in volume by 20cm3 means that total volume of product is less than total volume of reactant.

(10 + 20x + 5y) - (10x + 5y) = 20
10x + 10 = 20
x = 1

now try to work out y.
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Akib Raihan
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At room temp. and pressure there shouldn't have been any water vapour .
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(Original post by Akib Raihan)
At room temp. and pressure there shouldn't have been any water vapour .
oh yes,
in that case, we need to eliminate the water bit from the equation and we try different general formula. also realised another error on equation it shud be:

CxHy + (x + y/4) O2 => xCO2 + y/2H2O
try different general formula.
1) where y = 2x + 2 for alkane:
10 + 10(0.5x+0.5 + x) - 10x = 20
10 + 5x + 5 + 10x - 10x = 20
5x + 15= 20
x = 1
so CH4 is viable

try y = 2x
10 + 10(0.5x + x) - 10x = 20
10 + 5x + 10x -10x = 20
5x + 10 = 20
x = 2
so C2H4 is viable.
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Akib Raihan
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Answer is C4H4 😅
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username3718068
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Oh dear not again...
it is MCQ?
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(Original post by Akib Raihan)
Answer is C4H4 😅
wait you can still get C4H4!
refer back to eqn...

now try y = x

10 + 10(x + x/4) - 10x = 20
10 + 10x + 2.5x - 10x = 20
10 + 2.5x = 20
x = 4
hence y = 4
so C4H4... not exactly the ideal method, but certainly not an incorrect one either.
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Akib Raihan
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yes its MCQ . but why did assume that y=x ??
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