Finding Empirical Formula And Molecular Formula Of Compounds

Empirical formula is a formula that shows the simplest ratio in which elements are combined in a compound. For example, the empirical formula of a compound that has a chemical formula of C2H6 is CH3 because carbon and hydrogen are combined in the ratio 1:3.

Empirical formula can be calculated using either of the following:

  • the actual masses of the combined elements.
  • the percentage composition by mass of the combined elements.
  • experimental data from a chemical reaction.

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Calculating empirical formula of a compound given the masses of combined elements

Given the masses of the combined elements in a compound, we can calculate the empirical formulae of the compound as follows:

  • Change the given masses to moles of their atoms by dividing the masses with molar mass of the atoms.
  • Find the simplest molar ratio of the combined atoms.
  • Use the molar ratio to write down the empirical formula.

Example: 56 g of iron combine with 32 g of sulphur to form iron sulphide. Find the empirical formula for iron sulphide.

To make the calculation easier, you tabulate the data as follows:

IronSulphur
mass56 g32 g
find the number of moles56/56 = 132/32 = 1
express the moles as a simplest ratio11

Since the ratio of Fe:S is 1:1, the empirical formula is FeS.


Empirical formula of a compound given the percentage composition by mass of the combined elements.

Given the percentages by mass of the combined elements in a compound, we can calculate the empirical formulae of the compound as follows:

  • Treat the percentages as masses
  • Change the given masses to moles of their atoms by dividing the masses with molar mass of the atoms.
  • Find the simplest molar ratio of the combined atoms.
  • Use the molar ratio to write down the empirical formula.

Example: An oxide of sulphur contains 40% sulphur and 60% oxygen by mass. What is its empirical formula?

When given percentages we treat 100% as 100 g, which means the mass of sulphur becomes 40 g and the mass of oxygen 60 g.

SulphurOxygen
mass40 g60 g
find the number of moles40/32 = 1.2560/16 = 3.75
express the moles as a simplest ratio by diving with the smallest of the two number1.25/1.25 = 13.75/1.25 = 3

The simples ratio of sulphur : oxygen = 1:3

Therefore the empirical formula = SO3.

Calculating molecular formula, given empirical formula and Mr

When can easily convert empirical formula to molecular using the following formula:

n(empirical mass) = molecular mass, whereby n is the formula multiplier.

Determine the empirical formula of an organic hydrocarbon compound which contains 80% by mass of carbon and 20% by mass of hydrogen. If the Mr of the compound is 30, what is its molecular formula?

First we find the empirical formula using the given percentages:

carbonhydrogen
mass80 g20 g
find the number of moles80/12 = 6.6720/1 = 20
express the moles as a simplest ratio6.67/6.67 = 120/6.67 = 3

The simples ratio of carbon : hydrogen = 1:3

Therefore the empirical formula = CH3.

To find the molecular formula we first calculate the empirical mass the ratios given in the empirical formula

Empirical mass = (12× 1) + (1 × 3)

= 12 + 3

= 15

Then we find the formula multiplier

$latex n = \frac{Molecular\ mass}{Empirical\ mass}$

$latex n = \frac{30}{15}$

n = 2

Then multiply all the elements in the empirical formula by 2.

2(CH3) = C2H6

Therefore the molecular mass is C2H6

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