The empirical formula is the simplest whole-number ratio of atoms of each element present in a compound. It shows the relative number of atoms but not the exact number of atoms in a molecule. Chemists use empirical formulas to represent compounds in the simplest form and to understand the basic composition of substances.
Example: The molecular formula of Glucose is C6H12O6 , but its empirical formula is CH2O.
Molecular Formula
- The molecular formula shows the actual number of atoms of each element present in one molecule of a compound.
- It gives the exact composition of the substance.
Example:
The molecular formula of Glucose is:C6H12O6
This means that one molecule of glucose contains:
- 6 carbon atoms
- 12 hydrogen atoms
- 6 oxygen atoms
Thus, the molecular formula tells us the exact number of atoms in the molecule.
There is a relationship between molecular formula and empirical formula, which is given by,
Molecular Formula = n × Empirical Formula
where:
n is Whole Number
Calculating Empirical Formula
The empirical formula of a compound can be determined by finding the simplest whole-number ratio of the atoms of each element present in the compound. This is usually done using the percentage composition or mass of each element.
Steps to Determine Empirical Formula:
Step 1: Write the mass or percentage of each element
Start with the given mass or percentage composition of each element in the compound. If percentages are given, assume the total sample is 100 g so that the percentages can be treated as grams.
Step 2: Convert masses into moles
Convert the mass of each element into moles using the formula:
\text{Moles of element} = \frac{\text{Mass of element}}{\text{Atomic mass of element}}
Step 3: Find the simplest mole ratio
Divide all the mole values by the smallest number of moles to obtain the simplest ratio of the elements.
Step 4: Convert to whole numbers
If the ratios are not whole numbers, multiply them by a suitable number to convert them into whole numbers.
Step 5: Write the empirical formula
Use the whole-number ratio of atoms to write the empirical formula of the compound.
Empirical Formula | Molecular Formula |
|---|---|
| Shows the simplest whole-number ratio of atoms of each element in a compound. | Shows the actual number of atoms of each element in a molecule. |
| It represents the simplest form of a chemical formula. | It represents the exact composition of the compound. |
| It may not show the real number of atoms in the molecule. | It shows the true number of atoms present in the molecule. |
| One empirical formula can represent many molecular formulas. | One molecular formula represents only one specific compound. |
Example: For Glucose the empirical formula is CH2O. | Example: For Glucose the molecular formula is C6H12O6. |
Solved Examples
Example 1: The Empirical formula of Butane is C2H5. Calculate the Molecular formula when the measured mass of the compound is 58.1224 u.
Solution:
Atomic mass of given empirical formula = 2(C) + 5(H) = 2(12.011) + 5(1.00784) = 29.0612u
But, the measured molecular mass for Butane is given as 58.1224u
By using the expression, Molecular formula = n × empirical formula
n = molecular mass/weight of empirical formula
= 58.1224/29.0612
= 2
Molecular formula = 2 × C2H5
= C4H10
Example 2: Find a molecular formula for the compound having the empirical formula CH2 with a molecular weight of 42.08.
Solution:
Atomic mass of given empirical formula = C + 2(H) = 12.011 + 2(1.00784) = 14.02u
But, the measured molecular mass for given compound is 42.08u
By using the expression, Molecular formula = n × empirical formula
n = weight of molecular formula/weight of empirical formula
= 42.08/14.02
= 3
Molecular formula = 3 × CH2
= C3H6
Example 3: Find the empirical formula for the compound with molecular formula C6H12O6.
Solution:
C6H12O6 = 6 × CH2O
We know that Molecular Formula = n × Empirical Formula
Here, n = 6
So empirical formula for the given compound is CH2O.