# Hard AS calculation questions .. :/Watch

#1
1) 10cm3 of a hydrocarbon, CxHy were exploded with an excess of oxygen. There was a contraction in volume of 30cm3. When the products were treated with sodium hydroxide (which reacts with carbon dioxide), there was a further contraction of 30cm3. Deduce the formula of the hydrocarbon, given that all the volumes were measured under the same conditions.
So What I did so far:
CxHy +O2 -> CO2 + H2O
CxHy = 10 cm3
O2 =
CO2 =
H2O =
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#2
Bump!!
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3 years ago
#3
what spec is this for?
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#4
I'm doing Edexcel but found this question on the internet..
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#5
BUMP!!
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3 years ago
#6
Well, from the information given, you are told that 10cm3 of hydrocarbon was burnt.

10CxHy + some O2 → some CO2 + some H2O

You are told that the volume contracts by 30cm3
Note water turns into a liquid when burnt as it condenses when it climatises.

This means that 10cm3 + Some O2 - Some CO2 = 30cm3

You know you have 10cm3 of the hydrocarbon so:
Some O2 - Some CO2 = 20cm3

Hold on here! Volume further contracted by 30cm3 when Sodium Hydroxide reacted

This means that
Some O2 - 20cm3 = 30cm3 → Some O2 = 60cm3

You know have an equation.

10CxHy + 50 O2 → 30 CO2 + some H2O
Divide by 10 gives CxHy + 5O2 → 3CO2 + some H2O
You have 12 oxygens on one side and you know that 6 oxygens are used on the right hand side for 3CO2 so 4 oxygens are left for the H2O.

This gives 4H2O.
CxHy + 5O2 → 3CO2 + 4H2O
Having 3 carbons on the left side gives x = 3. You have 8 hydrogen atoms on the right hand side, so y = 8.

Final Formula C3H8
1
#7
(Original post by nwmyname)
Well, from the information given, you are told that 10cm3 of hydrocarbon was burnt.

10CxHy + some O2 → some CO2 + some H2O

You are told that the volume contracts by 30cm3
Note water turns into a liquid when burnt as it condenses when it climatises.

This means that 10cm3 + Some O2 - Some CO2 = 30cm3

You know you have 10cm3 of the hydrocarbon so:
Some O2 - Some CO2 = 20cm3

Hold on here! Volume further contracted by 30cm3 when Sodium Hydroxide reacted

This means that
Some O2 - 20cm3 = 30cm3 → Some O2 = 60cm3

You know have an equation.

10CxHy + 50 O2 → 30 CO2 + some H2O
Divide by 10 gives CxHy + 5O2 → 3CO2 + some H2O
You have 12 oxygens on one side and you know that 6 oxygens are used on the right hand side for 3CO2 so 4 oxygens are left for the H2O.

This gives 4H2O.
CxHy + 5O2 → 3CO2 + 4H2O
Having 3 carbons on the left side gives x = 3. You have 8 hydrogen atoms on the right hand side, so y = 8.

Final Formula C3H8
Oh I see!! Thank you soo much!!!
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