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Molecular Formula

What is Molecular Formula?

Last updated date: 21st Mar 2023
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Chemical formulas were first derived by determining the masses of all the elements that combine to create a molecule, leading to the development of two important types of chemistry formulae: molecular formula and empirical formula.

The molecular formula gives the exact number of every single atom present in a molecule, whereas the empirical formula gives the simplest ratio of the number of various atoms present. It is an empirical formula if the formula is simplified. The molecular formula is a multiple of the empirical formula and is often used.

What is Molecular Formula Meaning?

The molecular formula is a formula generated from molecules that represents the total number of individual atoms in a compound's molecule. A subscript in a molecular formula indicates the number of each type of atom in a molecule of the substance.

Molecular formulas are linked to gram molecular masses, which are simple whole number multiples of the empirical formula mass.

Relationship Between Empirical and Molecular Formula

The empirical formula is the simplest formula for a compound, defined as the ratio of subscripts of the smallest whole number of elements in the formula. It's also referred to as the most basic formula.

The formula of a substance expressed with the smallest integer subscript is called an empirical formula.

The empirical formula specifies the number of atoms in the compound in a given ratio. The empirical formula of a compound is directly proportional to its % composition.

What is Meant by Molecular Formula?

Molecular Formula  = n × Empirical Formula

This equation shows the molecular formula definition chemistry.

Solved Examples

Example 1: Caffeine has the following composition: 49.48% of carbon, 5.19% of hydrogen, 16.48% of oxygen and 28.85% of nitrogen. The molecular weight is 194.19 g/mol. Find out the molecular and empirical formula.

Solution:

Multiply percent composition with the molecular weight,

Carbon – 194.19 x 0.4948 = 96.0852

Hydrogen – 194.19 x 0.0519 = 10.07846

Oxygen –  194.19 x 0.1648 = 32.0025

Nitrogen – 194.19 x 0.2885 = 56.0238

Divide each value by the atomic weight,

Carbon : 96.0852 / 12.011 = 7.9997

Hydrogen : 10.07846 / 1.008 = 9.998

Oxygen : 32.0025 / 15.9994 = 2.000

Nitrogen : 56.0238 / 14.0067 = 3.9997

Round off the values to closest whole number,

8: Carbon

10: Hydrogen

2: Oxygen

4: Nitrogen

Hence, the molecular formula is C8H10N4O2.

As we know 2 is the common factor of 8, 10, 4 and 2.

The empirical formula is C4H5N2O.

Example 2: The empirical formula is BH3 of boron hydride. Calculate the molecular formula given the measured mass is 27.66.

Solution:

Atomic mass  = B + 3(H) = 10.81 + 3(1) = 13.81u

The measured molecular mass for an atom is given as 27.66u.

Molecular formula = n × empirical formula

n = molecular formula/empirical formula = 27.6613.81 = 2

Putting the value of n = 2 in the empirical formula,

Molecular formula = 2(BH3) = B2H6.

Conclusion

The masses of all the elements combine to form a molecule. The empirical formula gives a ratio of the number of atoms present, but the molecular formula offers the exact number of every single atom present in a molecule. The total number of individual atoms in a compound's molecule is represented by the molecular formula.

FAQs on Molecular Formula

Question: What is the Best Way to Write a Molecular Formula?

Ans: Divide the compound's molar mass by the empirical formula molar mass. The result should be a whole number or a close approximation of one. Multiply the whole number found in step 2 by all the subscripts in the empirical formula. The molecular formula is the end result.

Question: What is the Difference Between a Molecular Formula, Empirical Formula and Structural Formula?

Ans: Chemical formulas are divided into three categories: empirical, molecular, and structural. Molecular formula definition indicates the number of each type of atom in a molecule, and structural formulas indicate the atoms in a molecule are bound to each other. Empirical formulae show the simplest whole-number ratio of atoms in a compound, molecular formulas indicate the number of each type of atom in a molecule, and structural formulas explain how the atoms in a molecule are bound to each other.

Question: What are the Molecular Formula Examples?

Ans: Butene's chemical formula, C4H8, indicates that each freely existing molecule of butene includes four carbon atoms and eight hydrogen atoms. CH2 is its empirical formula. Two carbon atoms and four hydrogen atoms make up one molecule of ethylene (molecular formula C2H4).