2.4 empirical and molecular formula

Question 1

What is the empirical formula?

Answer 1

The simplest whole number ratio of each element in a compound.

Question 2

How do we work out the empirical formula?

Answer 2

1) Write out the elements involved
2) Write the percentages as masses
3) Divide these by their relative atomic masses to get the number of moles.
4) Divide all the numbers by the smallest number of moles.

Question 3

(Example) A compound contains 23.3% Magnesium, 30.7% Sulfur and 46.0% Oxygen. What is the empirical formula of this compound?

Answer 3

Mg = 23.3g / 24.3 (Ar) = 1

S = 30.7g / 32.1 (Ar) = 1

O = 46.0g / 16.0 (Ar) = 3

Empirical formula = MgSO₃

Question 4

What is the molecular formula?

Answer 4

The molecular formula is the actual number of atoms of each element in the compound.

Question 5

How do we work out the molecular formula from the empirical formula?

Answer 5

1) Firstly work out the empirical mass of the molecule
2) Divide the molecular mass by the empirical mass
3) Multiply the empirical formula by the number of empirical units

Question 6

(Example) The empirical formula of a molecule is CH₂O. It has a relative molecular mass (Mr) of 180. What is its molecular formula?

Answer 6

Empirical mass = 12 + (1 x 2) + 16 = 30
Mr = 180

Mr / empirical mass = 180 / 30 = 6 (empirical units)

CH₂O x 6 = C₆H₁₂O₆