# A level chemistry

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#1
15 cm3 of a gaseous hydrocarbon requires 90 cm3 of oxygen for complete combustion, both volumes being measured at 15 C and 1 atm. The formula of the hydrocarbon is
The answer should be C4H8 but don't know how, I have an idea that I should use ratio then use the same method of when we're trying to find Empirical formula but I tried and it is not working please help
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2 years ago
#2
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The moles of gaseous substances all occupy the same volume under the same pressure and temperature conditions.

So for volume measures you can substitute moles.

The question then simplifies to 15 moles of a hydrocarbon require 90 moles of oxygen.

Simplifying by dividing through by 15

1 mole of gaseous hydrocarbon requires 6 moles of oxygen.

You also know that all of the carbon in the hydrocarbon forms carbon dioxide, i.e. every mole of carbon in the hydrocarbon needs 1 mole of oxygen.

And every four moles of hydrogen requires one mole of oxygen as all of the hydrogen turns to water.

Now you look at different possibilities:

1 carbon atom would need a further 5x4 = 20 moles of hydrogen

CH20 is not a possible formula.

2 carbon atoms would need 4x4 = 16 atoms of hydrogen

C2H16 is not a possible formula.

3 carbon atoms would need 3x4 = 12 atoms of hydrogen

C3H12 is not a possible formula.

4 carbon atoms would need 2x4 = 8 atoms of hydrogen

C4H8 is a valid formula.
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