Can the molecular formula of a substance be determined from its percentage composition?

Answer 1

You can determine the empirical formula of a substance from its percentage composition. For example, a hydrocarbon is 75% C and 25% H. Calculate its empirical formula. Here is how to do it:

C75/12 H25/1 = C6.25 H25

(Notice that i have divided the % comp by atomic mass of respective element.)

Now divide throughout by smallest number:

C6.25/6.25 H25/6.25 = CH4

Answer 2

Yes but you need to know the molecular mass of the substance. Since the empirical formula is the simplest ratio its a reduced or smaller form of the molecular formula. If you know the mass of the molecule you can then see how many times bigger the molecular formula is over the empirical

ex. the first response gave up CH4 as the empirical formula the mass of this is 16amu. If the molecular formula has a mass of 32amu then you know that is is twice as big as the empirical and therefor the molecular formula is C2H8.

Answer 3

Yes, but not conclusively. You divide the percent compositions by the atomic masses to determine their relative number of atoms, the empirical formula. However, the compound hexane, C6H14, has an empirical formula of C3H7, because that's the simplest terms of the ratio. You still need to know a little bit about covalent bonding and molecular structure to figure out if the chemical formula is going to be the same as the empirical formula, or a multiple of it instead.

Answer 4

Divide the %age composition with its atomic mass.Do it for all elements. Establish a ratio of all the quotients.That is the empirical formula.Multiply the empirical formula with molecular mass/empirical mass(the mass of empirical formula).
eg. H2O * 36/18
= H2O * 2
= H4O2