A general formula is a formula that represents every member of a homologous series using the symbol n for the number of carbon atoms.
A general formula is a formula that represents every member of a homologous series, using the symbol n to stand for the number of carbon atoms. Malay: formula am · Chinese: 通式.
Each homologous series has its own general formula, and putting a whole number in place of n gives the molecular formula of a particular member. The alkanes follow CₙH₂ₙ₊₂, the alkenes follow CₙH₂ₙ, and the alcohols follow CₙH₂ₙ₊₁OH. Learning the general formula is a fast way to work out the formula of any member without drawing the molecule, and it is also how you check that a compound really belongs to the series you think it does.
Example. To find the third alkane, substitute n = 3 into CₙH₂ₙ₊₂: this gives C₃H₈, which is propane. Substituting n = 2 into the alkene formula CₙH₂ₙ gives C₂H₄, ethene. The same rule works for every member, however long the chain.
Confusion to avoid. Do not confuse the general formula with the molecular formula or the empirical formula. The general formula is the pattern with n in it, true for a whole series; the molecular formula is the actual count of atoms in one specific molecule; and the empirical formula is only the simplest ratio. For ethane the general formula is CₙH₂ₙ₊₂, the molecular formula is C₂H₆, and the empirical formula is CH₃.
The skill works just as well in reverse, and this is often where the marks are. Given an unfamiliar molecular formula, you can test which general formula it satisfies and so decide at a glance which family the compound belongs to. A compound with the formula that fits the alkene pattern must contain a double bond, and one that fits the alkane pattern must be saturated, so the general formula quietly carries information about structure and reactivity, not just about counting atoms. Being able to move confidently in both directions, from a value of n to a formula, and from a formula back to its series, is exactly the fluency that a well-organised answer shows.
The general formula ties directly to the idea of a homologous series, so mastering it early makes naming, deducing formulae and predicting reactions much quicker throughout the Carbon Compounds work that SPM Chemistry sets.
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