What is Empirical Formula? Steps For Determination

What is Empirical Formula?

The empirical formula of a compound is the simplest whole number ratio of each type of atom in a compound. It can be calculated from information about the mass of each element in a compound or from the percentage composition.

Visually, the empirical formula looks similar to the molecular formula, which gives the number of atoms in a single molecule of a compound. In fact, a compound’s empirical formula can end up being the same as its molecular formula, but this doesn’t always happen.

For example

Empirical formula of C₆H₁₂O₆ is CH₂O.

Empirical formula of C₆H₆  is CH.

Steps for Determination of Empirical Formula

The steps for determining the empirical formula of a compound are as follows:

Step 1: Determination of the percentage composition.

Step 2: Determine the number of gram atoms of each element. For this purpose divide the mass of each element (% of an element) by its atomic mass.

Step 3: Determination of the atomic ratio of each element. To get this, divide the number of moles of each element (gram atom) by the smallest number of moles.

Step 4: If the atomic ratio is a simple whole number, it gives the empirical formula. Otherwise, multiply with a suitable digit to get the whole number atomic ratio.

Solved example

Ascorbic acid (vitamin C) contains 40.92% carbon, 4.58% hydrogen, and 54.5% oxygen by mass. What is the empirical formula of ascorbic acid?

Solution

From the percentage of these elements, we believe that in 100 grams of ascorbic acid, there are 40.92 grams of carbon, 4.58 grams of hydrogen, and 54.5 grams of oxygen.

Divide these masses of the elements (or percentage) by their atomic masses to get the number of gram atoms.

Number of gram atoms of carbon = 40.92/12.0 gmol^–  = 3.41

Number of gram atoms of hydrogen = 4.58/1.008 gmol^– = 4.54

Number of gram atoms of oxygen  = 54.5/16 gmol^–  = 3.406

The atomic ratio is obtained by dividing the gram atoms by 3.406. which is the smallest number.

C:H:O  = 3.41/3.406 : 4.54/3.406 : 3.406/3.406

C:H:O  =    1 :3:1

To convert them into whole numbers, multiply by three

C:H:O  = 3(1 :3:1) = 3:4:3 This whole number ratio gives us the subscripts for the empirical formula of the ascorbic acid i.e., C₃H₄O₃

Uses of Empirical Formula

Here are the top 5 uses of empirical formulas in chemistry.

1. For predicting the properties of substances

One of the most important uses of empirical formulas in chemistry is to predict the properties of chemicals.

For example, it is possible to know the melting point of a particular compound by knowing its empirical formula.

As we know that some compounds are very toxic, poisonous, and volatile, and they need to be handled with extra caution. So, the empirical formula helps the chemist to understand the properties of such substances, and thus, it is possible to identify these substances and thus, the toxic ones.

2. To design new molecules

It is possible to know the structure of a molecule by knowing its empirical formula. For example, it is possible to know the structure of a particular compound by knowing its empirical formula. This is because the empirical formula tells us how many atoms are present in the compound.

3. To distinguish different kinds of materials

Chemists and researchers use empirical formulas to identify different materials. For example, it is possible to know that the material called ‘calcium oxide’ is composed of calcium and oxygen. By knowing the empirical formula, it is possible to know that it is an oxide of calcium.

4. In organic synthesis

In organic synthesis, the empirical formula is used to predict the structure of a compound. It is possible to predict the structure of a compound by knowing the empirical formula.

5. In inorganic chemistry

In inorganic chemistry, the empirical formula is used to understand the structure of the compound.

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