Molecular Weight Formula – Definition, Formula, Solved Examples

An atom is the basic building block of matter i.e. everything around us is made of matter. This is the smallest unit into which matter can be divided without the release of any electrically charged particles. This is the smallest unit of matter which has the characteristic properties of a chemical element. The mass of an atom depends on the electrons and neutrons present in the atoms. A number of atoms combine together to form a molecule. 

Molecules are made up of one or more than one atoms. The mass of a molecule depends on the different atoms present in the molecule. The mass of the molecule is known as the molecular mass. It is measured in Daltons. It is even referred to as molecular mass, molecular weight, or molar mass. Dalton is defined as the relative to the mass carbon-12 isotope. An atomic mass unit is described as the weight of the carbon-12 isotope divided by 12. The molecular mass of a material is important to know the number of grams present in one mole of the given substance. 

Molecular mass of some of the elements are:

Atomic No.	Name of  Element	Mass No.
1	Hydrogen	1
2	Helium	4
3	Lithium	7
4	Beryllium	9
5	Boron	10
6	Carbon	12
7	Nitrogen	14
8	Oxygen	16
9	Florine	19
10	Neon	20
11	Sodium	23
12	Magnesium	24
13	Aluminum	27
14	Silicon	28
15	Phosphorous	31
16	Sulfur	32
17	Chlorine	35.5
18	Potassium	39
19	Argon	40
20	Calcium	40

The molecular mass of a molecule is equal to the sum of the mass of each atom present in the molecule.

Molecular mass of compound = {(number of atoms in a compound)×atomic mass} for each atom.

Problems based on Molecular Mass

Problem 1: What is the molecular mass of Ethene?

Solution:

The molecular formula of ethene is C₂H₄

The atomic mass of carbon is 12 and the atomic mass of hydrogen is 1

So the molecular mass is (2×12) + (4×1)

                                      = 24 + 4

                                      = 28

Problem 2: What is the molecular mass of Methanol?

Solution:

The molecular formula of Methanol is CH₃OH

The atomic mass of carbon is 12, the atomic mass of hydrogen is 1 and the atomic mass of oxygen is 16

So the molecular mass is (1×12) + (4×1) + (1× 16)

                                     =12 + 4 + 16 

                                     =3

Problem 3: What is the molecular mass of Propane?

Solution: 

The molecular formula of Propane is C₃H₈

The atomic mass of carbon is 12 and the atomic mass of hydrogen is 1 

So the molecular mass is (3×12) + (8×1) 

                                    = 36 + 8 

                                    = 44

Problem 4: What is the molecular mass of Heptane?

Solution:

The molecular formula of Heptane is C₇H₁₆

The atomic mass of carbon is 12 and the atomic mass of hydrogen is 1

So the molecular mass is (7×12) + (16×1)

                                   = 84 + 16

                                   =100

Problem 5: What is the molecular mass of Ethane?

Solution: 

The molecular formula of Ethane is C₂H₆

The atomic mass of carbon is 12 and the atomic mass of hydrogen is 1

So the molecular mass is (2×12) + (6×1)

                                  = 24 + 6

                                  = 30

Problem 6: What is the molecular mass of Butanol?

Solution:

The molecular formula of Butanol is C₄H₈O

The atomic mass of carbon is 12, the atomic mass of hydrogen is 1 and the mass of oxygen is 16

So the molecular mass is (4×12) + (8×1) + (1×16)

                                  = 48 + 8 + 16

                                  = 72

Problem 7: What is the molecular mass of Benzoic acid?

Solution:

The molecular formula of Benzoic acid is C₆H₅COOH

The atomic mass of carbon is 12, the atomic mass of hydrogen is 1 and the atomic mass of oxygen is 16

So the molecular mass is (6×12) + (6×1) + (2×16)

                                  = 72 + 6 + 32

                                  =110

Problem 8: What is the molecular mass of Styrene?

Solution: 

The molecular formula of Methanol is C₈H₈

The atomic mass of carbon is 12 and the atomic mass of hydrogen is 1

So the molecular mass is (8×12) + (1×8

                                  = 84 + 8

                                  = 92

Problem 9: What is the molecular mass of Valine?

Solution: 

The molecular formula of Valine is C₅H₁₁NO₂

The atomic mass of carbon is 12 ,the atomic mass of hydrogen is 1, the atomic mass of oxygen and atomic mass of nitrogen is 14

So the molecular mass is (5×12) + (1×11) + (1×14) + (2× 16)

                                  = 60 + 11 + 14 + 32

                                  =117

Frequently Asked Questions

Question: Define a molecule.

Answer:

A number of atoms combine together to form a molecule. Molecules are made up of one or more than one atoms. The mass of a molecule depends on the different atoms present in the molecule. The mass of the molecule is known as the molecular mass.

Question: What is Molecule mass?

Answer:

The mass of the molecule is known as the molecular mass. It is measured in Daltons. It is even referred to as molecular mass, molecular weight, or molar mass. Dalton is defined as the relative to the mass carbon-12 isotope. An atomic mass unit is described as the weight of the carbon-12 isotope divided by 12. The molecular mass of a material is important to know the number of grams present in one mole of the given substance. 

Question: What are Molecular formulas and Empirical formulas?

Answer:

There are three main categories of chemical formulae: analytical, molecular, and structural. Empirical formulas are used to show the simplest ratio of atoms in a compound, whereas, molecular formulas show the exact number of each atom type in a molecule, and structural formulas also tell us how the atoms in a molecule are bonded together.