Empirical Formula (DP IB Chemistry): Revision Note
Empirical Formula
The molecular formula shows the actual number and type of atoms in a molecule:
E.g. the molecular formula of ethanoic acid is C2H4O2
The empirical formula gives the simplest whole-number ratio of atoms of each element in a compound
E.g. the empirical formula of ethanoic acid is CH2O
It can be determined from percentage composition by mass data
Organic compounds often have different empirical and molecular formulae
The formula of an ionic compound is always written as an empirical formula
Worked Example
Determine the empirical formula of a compound that contains 10 g of hydrogen and 80 g of oxygen.
Answer:
| Hydrogen | Oxygen |
---|---|---|
Note the mass of each element | 10 g | 80 g |
Divide the masses by atomic masses | 10 = 10 mol | 80 = 5 mol |
Divide by the lowest figure to obtain nearest whole number ratio | 10 =2 | 5.0 5.0 =1 |
Empirical formula | H2O |
Worked Example
Determine the empirical formula of a compound that contains 85.7% carbon and 14.3% hydrogen.
Answer:
| Carbon | Hydrogen |
---|---|---|
Note the X by mass of each element | 85.6 | 14.3 |
Divide the X by atomic masses | 85.7 =7.14 mol | 14.3 = 14.2 mol |
Divide by the lowest figure to obtain nearest whole number ratio | 7.14 = 1 | 14.2 = 2 |
Empirical formula | CH2 |
Molecular formula
The molecular formula shows the actual number of atoms of each element in a compound
To determine the molecular formula:
Divide the compound’s relative molecular mass (Mr) by the relative mass of the empirical formula
This gives a whole number multiplier
Multiply the empirical formula by this number to get the molecular formula
Worked Example
The empirical formula of X is C4H10S and the relative molecular mass of X is 180.42.
What is the molecular formula of X?
Relative Atomic Mass: Carbon: 12.01, hydrogen: 1.01, sulfur: 32.07
Answer:
Step 1: Calculate the relative mass of empirical formula
Relative empirical mass = (C x 4) + (H x 10) + (S x 1)
Relative empirical mass = (12.01 x 4) + (1.01 x 10) + (32.07 x 1)
Relative formula mass = 90.21
Step 2: Divide relative molecular mass of X by relative mass of empirical formula
Step 3: Multiply the empirical formula by 2
2 x C4H10S = C8H20S2
The molecular formula of X is C8H20S2
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