Question

How do you determine the molecular formula of a substance for its percentage composition?

Answer 1

The empirical formulae is the ratio between the numbers of atoms of different elements which is present in the molecule of the given compound. To calculate empirical formula from molecular formula we have to divide all the subscripts in the empirical formulae by the simplest whole number.

Complete step by step answer:
- Empirical formulas usually give the proportion of the elements present in the molecule of the given compound but nor the actual number of the atoms or how the atoms are arranged.
- Molecular formula is the actual whole number ratio of the elements present in the molecule of the compound.
Empirical formula is calculated from the molecular formula using the given steps:
Step 1 – write down the molecular formulae of the given compound.
Step 2- find the simplest whole number by which the compound must be divided to convert the molecular formulae into its simplest form.
- Example :
If compound contains elements X and Y in 1 : 4 mass ratio
Atomic masses of X and Y are in the ratio 1 : 2
Moles of X = $\dfrac{1}{1} = 1mol$
Moles of Y = $\dfrac{4}{2} = 2mol$
The ratio of moles of X and Y will be 1 : 2
The empirical formula will be: $X{{Y}_{2}}$

Note: If mass of compound is given then empirical formula is calculated by changing the mass of every element to moles and then by dividing every mole value by the lowest value of mole computed and then the value of mole is rounded off to the closest whole number.