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    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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    Bump!!
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    what spec is this for?
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    I'm doing Edexcel but found this question on the internet..
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    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
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    (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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