Alkanes
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A hydrocarbon contains 14.3% hydrogen by mass. Three isomeric forms exist whose boiling points are 6 oC, 1 oC, and 4 oC. How do you deduce the molecular formula of the hydrocarbon?
Corrie writes ...
If 100 g of the hydrocarbon contains 14.3 g of hydrogen, then it must also contain 85.7 g of carbon. Therefore, in moles, it will contain 14.3/1 = 14.3 mol H and 85.7/12 = 7.14 mol C. This is a C/H ratio of 1:2, so the empirical formula must be CH2.
The molecular formula could therefore be CH2, or C2H4, or C3H6, etc
In the absence of the molecular mass or a data source giving boiling points of hydrocarbons to identify the unknown, you will have to deduce its molecular formula by considering the number of isomers for each of the possible molecular formulas in turn. You can rule out CH2 and C2H4 straight away as they have no isomers (and CH2 is not a stable molecule anyway).
So start by considering the possible isomers of a hydrocarbon with the formula C3H6. If this doesn't fit, then consider C4H8, and so on, until you find one with three suitable isomers.
Risk assessment
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For further guidance see our tutorial on Risk Assessment.
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updated: 09 September 2007
