Calculating Empirical Formula
- The empirical formula is the simplest whole number ratio of the atoms of each element present in one molecule or formula unit of the compound
- E.g. the empirical formula of ethanoic acid is CH2O
- Organic molecules often have different empirical and molecular formulae
- The formula of an ionic compound is always an empirical formula
A compound that contains 10 g of hydrogen and 80 g of oxygen.
What is its empirical formula of this compound?
|Write the mass of each element||10||80|
|Divide each mass by the relative atomic mass||10/ 1= 10||80/ 16= 5|
|Find the ratio by dividing by the smallest number||10/5 = 2||5/5= 1|
Empirical formula = H2O
If you don't get a whole number when calculating the ratio of atoms in an empirical formula, such as 1.5, multiply that and the other ratios to achieve whole numbers.
Calculating Molecular Formula
- Molecular formula gives the actual numbers of atoms of each element present in the formula of the compound
- To calculate the molecular formula:
- Step 1: Find the relative formula mass of the empirical formula
- Step 2: Use the following equation:
- Step 3: Multiply the number of each element present in the empirical formula by the number from step 2 to find the molecular formula
Table showing the Relationship between Empirical and Molecular Formula
The empirical formula of X is C4H10S1
The relative formula mass (Mr ) of X is 180.
What is the molecular formula of X?
(Relative atomic mass: Carbon : 12 Hydrogen : 1 Sulfur : 32 )
Step 1 - Calculate the relative formula mass of the empirical formula
Mr = (12 x 4) + (1 x 10) + (32 x 1) = 90
Step 2 - Divide relative formula mass of X by relative formula mass of empirical formula
180 / 90 = 2
Step 3 - Multiply each number of elements by 2
(C4 x 2) + (H10 x 2) + (S1 x 2)
Molecular Formula of X = C8H20S2