Molecular Mass

Molecular Mass is the mass of all the atoms present in a molecule. In ancient India and Greece, philosophers have first given the idea of atoms and deeply studied them. Around 500 BC.

Everything around is made up of very small units these units are atoms in the language of science, very small in the sense is in the range of 0.1 – 0.5 nanometers. These atoms join to form molecular which then join to form matter. For example, two atoms of hydrogen and one atom of oxygen joins to form one molecule of H₂O which is water. Hence, the molecular mass in this case of H₂O will be given as sum of mass of two atoms of hydrogen and one atom of oxygen.

What is a Molecule?

A molecule is a group of two or more atoms that are chemically bonded together, i.e, tightly held together by attractive forces. A molecule can be defined as the tiniest particle of an element or a compound that is fit for a free existence and shows all the properties of its parent substance. 

Atoms of the same or of different elements can form molecules. Following is the list of some important molecules with their combining ratios and mass ratios as:

Compound	Combining Elements	Ratio of Elements by Number of Atoms	Ratio of Elements by Mass
Water	Hydrogen & Oxygen	2:1	1:8
Ammonia	Nitrogen & Hydrogen	1:3	14:3
Carbon Dioxide	Carbon & Oxygen	1:2	3:8
Methane	Carbon & Hydrogen	1:4	3:1
Glucose	Carbon, Hydrogen & Oxygen	1:2:1	6:1:8

Mole Concept

Mole concept states that it is easier to use the quantity of substance in a number of molecules or atoms instead of their mass so the unit called Mole is used.

If the element is weighed and it is equal to its atomic weight in grams, then it contains a certain amount of element, which represents one mole of an element, and that certain amount of element is 6.022 x 10²³ atoms of an element. 1 mole of any molecule or atom is equal to its atomic mass or molecular mass in grams. The number of particles in 1 mole = 6.022 x 10²³ atoms or molecules and this is known as the Avogadro number.

The number of particles in 1 mole: 

• 1 mole of hydrogen atoms shows 6.022 × 10²³ hydrogen atoms.
• 1 mole of hydrogen molecules shows 6.022 × 10²³ hydrogen molecules.
• 1 mole of water molecules shows 6.022 × 10²³ water molecules.

Mathematically, the number of moles is defined as:

Example 1: Find the number of moles present inside the 24.088 ×10²³ particles of water.

Answer:

Given that, 

The number of particles present, N = 24.088 ×10²³

The number of moles (n) = Number of particles (N) / Avogadro number (N₀)

                                         = 24.088 ×10²³ / 6.022 x 10²³

                                         = 4 moles

What is Molecular mass?

The molecular mass of an element is defined as the sum of all the masses of the elements present inside the molecule. e.g. the molecular mass of water, in which there are two hydrogen atoms and one oxygen atom present, is 18 (i.e. 2 + 16). Also called its molecular weight.

Unit of Molecular Mass

SI unit of Molecular Mass or Molecular Weight is an atomic mass unit which in short is called a.m.u. It is sometimes represented by u which represents unified mass.

Molecular Mass of some Elements

Molecular mass is predicted by multiplying the number of atoms with the atomic mass of an element in the molecule and then adding the masses of all the elements in the molecule. The following table shows the atomic mass of the first twenty elements of the periodic table as,

Name	Atomic number	Symbol	Atomic Weight (in a.m.u.)
Hydrogen	1	H	1.00979
Helium	2	He	4.0026
Lithium	3	Li	6.941
Beryllium	4	Be	9.01218
Boron	5	B	10.81
Carbon	6	C	12.011
Nitrogen	7	N	14.0067
Oxygen	8	O	15.9994
Fluorine	9	F	18.998403
Neon	10	Ne	20.179
Sodium	11	Na	22.98977
Magnesium	12	Mg	24.305
Aluminium	13	Al	26.98154
Silicon	14	Si	28.0855
Phosphorus	15	P	30.97376
Sulphur	16	S	32.06
Chlorine	17	Cl	35.453
Argon	18	Ar	39.0983
Potassium	19	K	39.948
Calcium	20	Ca	40.08
Scandium	21	Sc	44.9559
Titanium	22	Ti	47.9
Vanadium	23	V	50.9415
Chromium	24	Cr	51.996
Manganese	25	Mn	54.938
Iron	26	Fe	55.847
Cobalt	27	Co	58.9332
Nickel	28	Ni	58.7
Copper	29	Cu	63.546
Zinc	30	Zn	65.38

What are Ions?

It is an electrically charged species or atom or group of atoms. It is formed by either gaining or losing one or more electrons by an atom.

Ions are of two types:

Cation

Cations are formed by the loss of one or more electrons from an atom, so they are positively charged ions. e.g.

• On losing one electron sodium atom forms a sodium ion Na as,

Na – e⁻  →  Na⁺

• On losing two electrons Magnesium forms a magnesium ion as,

Mg – e^– → Mg⁺

          Again it loses one more electron to complete its octet as, 

Mg⁺ – e^–  → Mg²⁺

Anion

It is a negatively charged ion and is formed by the gain of one or more electrons by an atom. e.g.

• On gaining one electron chlorine form a chloride ion Cl⁻ as, 

Cl + e⁻  →  Cl⁻

• On gaining two electrons oxygen form an oxide ion as,

O  +  e^–  → O^–  

          Again it gains one more electron to complete its octet as, 

O^– + e- →  O^2-

Calculation of Molecular Mass

Molecular mass is predicted by multiplying the number of atoms with the atomic mass of an element in the molecule and then adding the masses of all the elements in the molecule.

1. Make sure to analyze the chemical formula to determine the number of atoms of each element in the compound.
2. Multiply the number of atoms present in the compound by the atomic weight of each element.
3. Add up all the mass and assign its unit as grams/mole.

Example 1: Determine the Molar mass of CO₂.

Answer: 

First Calculate the number of each atom, and assign their atomic weight individually

The number of carbon atoms × its atomic mass: 1 × 12 = 12

The number of oxygen atoms × its atomic mass: 2 × 12 = 24

Then add the values i.e., 12 + 24= 36

Therefore the molar mass of CO₂ is 36 g/mol.

Example 2: Determine the formula unit mass of CaCl₂.

Answer:

The formula unit mass of CaCl₂ = Atomic mass of Ca + 2 × (Atomic mass of Cl) 

formula unit mass of CaCl₂ = 40 + 2 × 35.5 

formula unit mass of CaCl₂ = 40 + 71 

formula unit mass of CaCl₂ = 111 a.m.u.

Solved Examples of Molecular Mass

Example 1: What is an Atomicity? List the atomicity of different main elements of the periodic table.

Answer:

Atomicity is defined as the number of atoms present in one molecule of an element or a compound is known as its atomicity. below is the table that shows the atomicity.

Name of the Element	Atomicity
Argon	Monoatomic
Helium	Monoatomic
Oxygen	Diatomic
Hydrogen	Diatomic
Nitrogen	Diatomic
Chlorine	Diatomic
Phosphorus	Tetra-atomic
Sulphur	Poly-atomic

Example 2: What is the molar mass of sodium carbonate, Na₂CO₃?

Answer:

Since, a molecule of sodium carbonate contains two atoms of sodium (Na), one atom of carbon (C) and three atoms of oxygen (O). The molecular weight would be

Na: 2 × 23.0 g =  46 g

C: 1 × 12.0 g = 12 g

O: 3 × 16 g = 48 g

When we add up the total values i.e, 46 g + 12 g + 48 g = 106 g

Hence, the molar mass of Na₂CO₃ is 106 g/mole.

Example 3: Calculate the molar mass of H₂SO₄.

Answer:

The molar mass can be calculated as:

Molar mass of H₂SO₄  = (2 × Mass of Hydrogen) + (1 × Mass of Sulphur) + (4 × Mass of Oxygen)

                                   =  (2 × 1 + 1 × 32 + 4 × 16) g/mol

                                   = (2 + 32 + 64) g/mol

                                   = 98 g/mol

Example 4: State the number of hydrogen (H) atoms in 1 g of Hydrogen.

Answer: 

Since one gram of Hydrogen (H) contains 1 mole of Hydrogen which is equal to 6.022 x 10²³ atoms.

Example 5: How many atoms are present in: 

(a) H₂S molecule and 

(b) PO₄⁻³ ion?

Answer:

(a) In H₂S, there is one sulphur and two hydrogen atoms present so a total of 3 atoms are present.

(b) In PO₄⁻³, one Phosphorus and 4 oxygen atoms are present so a total of 5 atoms are present.

Example 6: What is the mass of:

(a) 1 mole of nitrogen atoms?

(b) 4 moles of aluminium (Al) atoms (Atomic mass Al= 27)?

(c) 20 moles of sodium sulphite (Na₂SO₃)?

Answer:

(a) 1 mole of Nitrogen = 1 × Atomic mass of nitrogen atom

                                     = 1 × 14 g 

                                     = 14 g

(b) 4 moles of aluminium atoms = 4 × Atomic mass of aluminium atom

                                                    = 4 × 27 g 

                                                    = 108 g

(c) 20 moles of sodium sulphite (Na₂SO₃) = 20 (2 × Atomic mass of Na + 1 × Atomic mass of sulphur + 3 × Atomic mass of oxygen)

                                                                   = 20 (2 × 23 g+1 × 32 g + 3 × 16 g)

                                                                   = 20 (46 g + 32 g + 48 g)

                                                                   = 20 × 126 g 

                                                                   = 2520 g

FAQs on Molecular Mass

What is molecular mass?

The molecular mass of an element is defined as the sum of all the masses of the elements present inside the molecule. e.g. the molecular mass of water, in which there are two hydrogen atoms and one oxygen atom present, is 18 (i.e. 2 + 16). 

What is the molecular mass of glucose?

Chemical formula for glucose is C₆H₁₂O₆

Thus, molecular mass of glucose = 6×12 + 12×1 + 6×16 = 72 + 12 + 96 = 180

What is the molecular mass of oxygen?

Molecular Mass of Oxygen is approximately 16 gm/mole.

How many atoms are in one Mole of substance?

There are same as Avagadro number i.e., 6.022×10²³ atoms in the one mole of substance.