Empirical Formula & Molecular Formula

Initially, chemical formulas were obtained by determination of masses of all the elements that are combined to form a molecule and subsequently we come up with two important types of formulas in chemistry: molecular formula and empirical formula.

The empirical formula of a compound gives the simplest ratio of the number of different atoms present, whereas the molecular formula gives the actual number of each different atom present in a molecule. If the formula is simplified then it is an empirical formula. The molecular formula is commonly used and is a multiple of the empirical formula.

The general statement relating molecular formula and the empirical formula is

Molecular Formula  = n \(\times\) Empirical Formula

What is Molecular formula?

  • The molecular formula is the formula derived from molecules and is representative of the total number of individual atoms present in a molecule of a compound.
  • A molecular formula uses a subscript that reports the actual number of each type of atom in a molecule of the compound.
  • Molecular formulas are associated with gram molecular masses that are simple whole number multiples of the corresponding empirical formula mass.

What is Empirical formula?

  • The empirical formula is the simplest formula for a compound which is defined as the ratio of subscripts of the smallest possible whole number of the elements present in the formula. It is also known as the simplest formula.
  • An empirical formula for a compound is the formula of a substance written with the smallest integer subscript.
  • The empirical formula gives information about the ratio of numbers of atoms in the compound. The percent composition of a compound directly leads to its empirical formula.

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Empirical Formula Vs Molecular Formula

Empirical Formula Molecular Formula
An empirical formula represents the simplest whole-number ratio of various atoms present in a compound. The molecular formula shows the exact number of different types of atoms present in a molecule of a compound.
Example: For Acetylene the empirical formula is CH Example: For Acetylene the empirical formula is C2H2

Example (Glucose Molecular Formula Vs Glucose Empirical Formula)

Let’s take the example of glucose. The molecular formula of glucose is C6H12O6 and the empirical formula of glucose is CH2O. We can derive a relation between the Molecular formula and the empirical formula of glucose.

Empirical Formula & Molecular Formula of Butane & Octane[/caption]

C6H12O6 = 6 × CH2O

We can derive a general expression as,

Molecular formula = n × empirical formula where n is a whole number

Sometimes, the empirical formula and molecular formula both can be the same.

Solved Examples

Question-1:

The empirical formula of Boron Hydride is BH3. Calculate molecular formula when the measured mass of the compound is 27.66.

Solution

The atomic mass is given by = B + 3(H) = 10.81 + 3(1) = 13.81u

But, the measured molecular mass for Boron atom is given as 27.66u

By using the expression, Molecular formula = n × empirical formula

n = molecular formula/empirical formula = \( \frac {27.66}{13.81}\) = 2

Putting value of n = 2 in the empirical formula we get molecular formula as

Molecular formula = 2(BH3) = B2H6.

Question-2:

The empirical formula of a compound is COCl2 and its molecular mass is 90.00u. Find out the molecular formula of that compound.

Solution

COCl2 = C + O + 2(Cl) = 12 + 16 + 2(35.5) = 99 u

Empirical formula is same as molecular mass as n=1, this means molecular formula is COCl2.

Question-3:

What is the molecular formula of a compound which has an empirical formula of CH2 and a relative molecular mass of 70?

Solution

Relative molecular mass = 70
Empirical formula mass = 12 + 2 = 14
The relative molecular mass is 5 x the relative empirical formula mass
The molecular formula is 5 x the empirical formula

The molecular formula is C5H10

Frequently Asked Questions – FAQs

Why is it called empirical formula?

If no subscription exists, this means that one atom is present in the compound. The most straightforward formulation is also known as the analytical formula. The mathematical formulation is the ratio of the compound elements present. The subscripts in the formula are the numbers of atoms, resulting in a ratio of whole numbers between them.

Why do we use empirical formula?

Empirical formulas represent the simplest notational form. They provide the lowest ratio of whole numbers between the compound elements. They will not have information on the total number of atoms in a single compound molecule, as opposed to molecular formulae.

What is the formula for empirical formula?

Using the molar mass from the periodic table convert the mass of each element to moles. Divide each mole value by the smallest calculated number of moles. Round to complete number nearest. This is the mole ratio of the elements, and is represented in the empirical formula by subscriptions.

What is molecular formula and empirical formula?

Three main categories of chemical formulae exist: analytical, molecular and structural. Empirical formulas show the simplest total-number ratio of atoms in a compound, molecular formulas show the number of each atom type in a molecule, and structural formulas show how the atoms in a molecule are bonded together.

Why is empirical rule useful?

The empirical rule is of primary use in most cases to help determine outcomes when not all the data are available. It allows statisticians – or those studying the data – to gain insight into where, once everything is available, the data will fall. The empirical rule also aids in testing how normal a set of data is.

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