Chemical quantities (AS)
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How does relative formula mass differ from relative molecular mass, and how can it be used to calculate the molecular formula.
Ulex writes
There is little difference between these two terms. If the substance concerned has molecules (e.g. CO2), we would refer to ‘molecular mass’; if it consists of some other kind of particle(s) (e.g. NaCl, which consists of (+) and (-) ions), we might say ‘formula mass’.
Suppose we know from analysis that a hydrocarbon has 12 g of carbon combined with 1 g of hydrogen. Its simplest (empirical) formula is CH (mole ratio of C to H is 1:1). But if we also know that its relative formula mass is 78, its molecular formula must be not CH, but C6H6. The mass of CH only adds up to 13, and 78/13 is 6.
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updated: 14 August 2007
