I was recently taking a session and one of the students asked the question what is the significance of an Empirical formula and why we use it if it is not the true formula of the compound. This got me thinking as I did not have an off the cuff answer for this question. I made a mental note to explore this question further for the student and came up with the following reasons.
The composition of any compound is expressed as a chemical formula using symbols, numbers and sometimes signs like + or -. Chemical formulas are of three types – Molecular formula, Empirical formula, and structural formula.
Molecular formula and Empirical formula are quite similar as they represent the elemental composition of a compound. Whereas, a structural formula represents the structure of the compound and hence is different from the other two.
The Molecular formula is the chemical formula of a compound which shows, the elements with the total number of atoms present in each molecule of that compound.
the formula is the simplest whole number ratio of atoms of each element in a compound. Or, in simple words, it is the simplest chemical formula of a compound.
Given in the table below are few compounds with their molecular formula and empirical formula.
Difference between Empirical Formula and Molecular Formula.
The Empirical formula of a compound gives only the ratio of elements present in a compound. But, it does not give the actual number of atoms present in that compound. Due to this, an empirical formula is not the true formula of a compound.
Besides, any two compounds can have the same empirical formula. This is one more reason for not considering empirical formula as the molecular formula. Consider these examples.
Glucose, ribose, acetic acid and formaldehyde, all have different molecular formulas but the same empirical formula. Similarly, the compounds Sulphur monoxide and Disulphur dioxide share the same empirical formula but their molecular composition is different.
Elemental analysis of an unknown sample will not stop at a determination of empirical formula because of this ambiguity. It is always followed by the determination of the molecular formula and further confirmation by specific tests.
Molecular formula, on the other hand, gives the actual number of atoms of each element present in a compound and hence is the true formula of the compound. Sometimes, a molecular formula of a compound can be the same as empirical formula, as in the case of Sulphur monoxide and Formaldehyde, or can be the multiple of the empirical formula.
When the difference between molecular formula and empirical formula are clear, there’s a question that comes to the mind.
If the empirical formula does not give us the true identity of the compound, then why do we use it?
The Empirical formula tells us the ratio of elements in a compound which is helpful in
1. Calculating the percentage composition of the elements in a compound.
2. In an elemental analysis of an unknown sample.
3. In the synthesis of a new compound.
How to determine the empirical formula of a compound?
The empirical formula of a compound is determined by using
1. Molecular formula
2. Mass percentage data
Mass percentage data is used to determine the empirical formula when the molecular formula of the
compound is not known or when an unknown sample is analyzed.
Using Molecular formula
Determination of empirical formula of Glucose using molecular formula.
1. Write the molecular formula of the compound.
2. Divide the subscripts of the compound by the greatest common factor.
3. Use the values in student molecular formula.
C1H2O1 or CH2O
Using Mass Percentage Data
Determination of empirical formula of the compound which has 40% C, 6.71% H, and 53.33% O
using % composition of elements.
1. Assume 100g of a compound and convert a mass percentage of elements into grams.
So, 100g of a sample will have 40g of Carbon, 6.71g of Hydrogen and 53.33g of Oxygen.
2. Calculate the molar ratios of each element in the compound.
[Atomic mass of elements is considered – one mole of an element will always be equal to its gram atomic mass]
For Carbon, 1 mole of Carbon is 12g. So, 40g of Carbon will be
40* 1mole of C /12g of C = 3.33
For Hydrogen, 6.71* 1 mole of H/1g of H = 6.71
For Oxygen, 53.33 of O * 1 mole of O/16g of O = 3.33
So, we get the molar ratios of C, H, and O as 3.33, 6.71 and 3.33 respectively.
3. Divide the values by the lowest mole ratio.
The lowest mole ratio among the three values is 3.33.
Carbon 3.33/3.33 = 1
Hydrogen 6.71/3.33 = 2.01
Oxygen 3.33/3.33= 1
So, we get the molar ratios of C, H, and O as 1:2:1. Use this data to write the empirical formula.
4. Assign the empirical formula
The empirical formula of a given compound is C1H2O1 or CH2O
So, we have now determined the empirical formula of the given unknown sample using the mass percentage data. The next step is the determination of the molecular formula using the empirical formula.
We already know that an empirical formula is not the true formula of the compound. A group of compounds may share the empirical formula. But, now we know which group the sample belongs to and what specificity tests need to be performed.