Avogadro's number explained with worked examples

len Alfred Ajibola - 29th August, 2022 @ 11:00 PM

Topics in Chemistry

Avogadro's number explained with worked examples Mole and Avogadro's Number explained Le Chatelier's Principle: Changes in concentration and pressure in dynamic equilibrium Chemical Equilibrium: Dynamic Equilibrium in Chemistry Static and Dynamic Equilibrium explained with their differences Chemistry Scheme of Work, SS1, First Term Chemistry Scheme of Work, SS1, Second Term Chemistry Scheme of Work, SS1, Third Term Compounds in Chemistry: Characteristics of Compounds Types of Mixture: Homogenous and Heterogeneous Mixtures What are mixtures? Characteristics of mixtures Physical properties of matter Chemical properties of matter explained with examples Differences between physical and chemical change in chemistry Ionic Theory and Electrolysis Physical and Chemical Properties of Acids States of Matter: Characteristics of Solids Liquid state of matter: Characteristics of liquids Gaseous state of matter: Characteristics of gases Molar Mass, worked examples and differences with molecular mass

Academic Questions in Chemistry

Please click here to see all Questions and Answers

Which of the following isn't an element of the periodic table.

  • A. J

  • B. Y

  • C. W

  • D. B

  • E. U

  • F. K

According to the periodic table, elements in the same group have got the same number of shells.

  • A. True

  • B. False

The periodic table of elements is also called the Mendeleev's table.

  • A. True

  • B. False

The simplest formula of a compound is termed _____.

  • A. Emperical formula

  • B. Molecular formula

  • C. Avogadro's number

  • D. Mass formula

  • E. Molar mass formula

  • F. Mole ratio

A compound composed of iron (Fe) and oxygen (O) was analyzed and found to contain 69.94% iron and 30.06% oxygen. Find the empirical formula of the compound. (Molar mass of Fe=55.85, O=16)

  • A. FeO

  • B. Fe2O3

  • C. Fe3O4

  • D. FeO2

  • E. Fe3O6

  • F. Fe2O4

Which of the following statement is false concerning hydrocarbons?

  • A. Hydrocarbons can be aromatic

  • B. Hydrocarbons are made up of carbon and hydrogen only

  • C. Alkynes are aliphatic hydrocarbons

  • D. Alkenes have a carbon-carbon double bond in their structure

  • E. Benzene ring has a carbon-carbon triple bond in its structure

  • F. Hydrocarbons are the main constituents of petroleum and natural gas

The ability of carbon to form long chain of itself is called _____.

  • A. Carbon chain reaction

  • B. Catenation

  • C. Carbontion

  • D. Carbontion

  • E. Organic chemistry

  • F. Carbon moleculation

Calculate the relative molecular mass of methane. (Carbon = 12.001, H = 1.00794).

  • A. 13.00894 amu

  • B. 11.00206 amu

  • C. 12.00794 amu

  • D. 14.001 amu

  • E. 15.001 amu

  • F. 16.033 amu

What is Avogadro's Number?

Avogadro's number refers to the number of particles or atoms or molecules present in one mole of a substance. This number is 6.02214076 × 1023. It is termed Avogadro's constant.

The calculations involving Avogadro's number may require us to find the number of particles, atoms or molecules. We should not be confused on these terms as they all point to the same thing. However, we may further be asked to find the moles, mass or molar mass of a substance in questions related to Avogadro's numbers.

Please read on molar mass here


Questions related to Avogadro's number, mole and molar mass will be solved in this article. Meanwhile, an understanding on the concept of mole and Avogadro's number is required.

Please read an introduction to mole and Avogadro's number here


Questions and Answers on Avogadro's Number

  • Question 1

2H2(g) + O2(g) -> 2H2O(l)

From the above equation, calculate the number of atoms present in the reactants. (Avogadro's number is 6.02 × 1023).

  • Solution

The reactants present in the reaction are two moles of hydrogen molecules and one mole of oxygen molecule.

Worthy of note is the number present in front of the molecules involved in the reaction. This number is the mole itself. Hydrogen has 2 in front (that is, 2 moles of hydrogen) while oxygen has nothing, and this signifies 1 mole of oxygen. Water is the product from the reaction. It has 2 anteriorly and can be appropriately referred to as 2 moles of water.

Recall that one mole of any substance = 6.02 × 1023. These substances can be an atom, a molecule or a compound.

  • From the question, two moles of hydrogen = 2 x 6.02 × 1023 = 12.04 × 1023

  • One mole of oxygen = 1 x 6.02 × 1023 = 6.02 × 1023

Therefore, the reactant 2H2(g) contains 12.04 × 1023 atoms while O2(g) contains 6.02 × 1023

You can read on molecular mass and its calculations here


  • Question 2

How many moles are present in 120.4 × 1023 molecules. (Avogadro's number  is 6.02 × 1023).

  • Solution

Recall that one mole of any substance will always contain 6.02 × 1023 atoms, particles or molecules. The question uses 'molecules' in this instance, but regardless, it is still the same as atoms or particles.

  • If 1 mole = 6.02 × 1023

  • Then Z moles = 120.4 × 1023

  • Cross multiply

  • Z x  6.02 × 1023 = 1 x 120.4 × 1023

  • Make Z the subject of the formula

  • Z = 120.4 × 1023 / 6.02 × 1023

  • Z = 20 moles

Therefore 20 moles of Z will contain 120.4 × 1023 molecules.

Please read on empirical formula here


  • Question 3

3.01 × 1023 particles of sodium reacted with chlorine to produce sodium chloride. How many moles of sodium reacted and what is the molar mass of sodium according to the reaction. (Sodium = 23, Avogadro's number = 6.02 × 1023).

  • Solution

One mole of any substance contains = 6.02 × 1023.

  • If 1 mole of sodium = 6.02 × 1023

  • Then Y mole = 3.01 × 1023

  • Cross multiply

  • Y x 6.02 × 1023 = 1 x 3.01 × 1023

  • Make Y the subject of the formula

  • Y = 3.01 × 1023 / 6.02 × 1023

  • Y = 1/2 mol

Therefore, 1/2 mol of sodium is equivalent to 3.01 × 1023 particles of sodium.

Molar mass of sodium = 23g/mol

  • Since 1/2 mole reacted, the molar mass of sodium that reacted will be 1/2 x 23g/mol

  • 11.5g/mol

You can read on Dalton's atomic theory and its modifications here

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Amazing facts in Chemistry

The only letters that failed to appear on the periodic table are letters:

J     &     Q

Gold and copper are the only two non-silvery colored metals.

Copper is the only metal that is naturally antibacterial. For this reason, some children utilize 'copper water bottles' in schools

Water freezes faster when it’s warm than when it’s cold

Most element in their pure state exists physically in different forms. For instance, pure carbon can exist as both diamond and graphite. This phenomenon is called allotropy

If you pour a handful of salt into a full glass of water, the water level will go down rather than overflowing the glass.

Similarly, if you mix half liter of water and half litre of alcohol, the total volume of the liquid will be les than one litre


In chemistry, hydrocarbons can be classified as either aliphatic or aromatic. Recently, both classifications of hydrocarbon were based on their structure rather than their origin.

Aliphatic hydrocarbons are put into three main groups according to the types of bonds they possess. These are:

  1. Alkanes

  2. Alkenes

  3. Alkynes

They are shown in the image below:

Alkanes, Alkenes and Alkynes - Len Academy

It's important to note the followings:

  • Alkanes have single bonds (only) in their structures.

  • Alkenes always have a carbon-carbon double bond present in their structure.

  • Alkynes always have a carbon-carbon triple bond present in their structure.


Aromatic hydrocarbons are classified into:

  • Arenes: They contain benzene ring as a structural unit. Below is the structure of a benzene ring.

Benzene Ring - Len Academy


  • Nonbenzenoid aromatic hydrocarbons: They possess special stability but lack a benzene ring as a structural unit.

Organic chemistry is the study of carbon and it's compounds.

Carbon is the focus of organic chemistry because it has a wide chemical diversity in the sense that it can combine with other carbon atoms to form a long chain of carbon molecule. This ability and process whereby carbon can form a long chain of itself is called catenation.

Please read the introduction to organic chemistry here

A major challenge encountered when calculating molecular mass is that it becomes difficult or impossible to calculate especially when the relative molecular mass of large molecules, polymers and macromolecules are involved.

Examples of large molecules (with indefinite molecular masses) include carbohydrates, cellulose and complex sugars.

The large molecules (above) have no specific chemical formula throughout their volume.

Please read more on carbohydrates and sugars here

Understand that Relative Molecular Mass prove to be useful only when we calculate substances with small and definite molecular sizes. This was proven through the modifications of Dalton's atomic theory.

Please read on Dalton's atomic theory and its modifications here.

Calculate the Relative Molecular Mass of methane (CH4). (Carbon=12.001 and H=1.00794)

  • Molecular Mass = Atomic mass of carbon + Atomic mass of Hydrogen

  • Methane is composed of one atom of Carbon and 4 atoms of Hydrogen.

  • 12.001 + (1.00794 x 4)

Molecular Mass of Methane = 16.033 amu

Please read more on molecular mass here

Calculate the Relative Molecular Mass of water (H2O). (Oxygen=15.9994 and Hydrogen=1.00794)

  • Molecular Mass = Atomic mass of Hydrogen + Atomic mass of Oxygen

  • Water contains 2 atoms of Hydrogen and 1 atom of oxygen.

  • (1.00794 x 2) + 15.9994

Molecular Mass of Water = 18.015 Da

Please read on emperical formula and its calculations here

Gold - Len Academy

Below are some facts about gold:

  • It is the only metal that has a gold (golden) color

  • Gold is present inside the earth crust in all the seven continents

  • Pure gold is soft and very stretchable (It can even be stretched into threads)

  • The Latin name of Gold is aurum which means 'shining dawn'. For this reason, its chemical symbol is 'Au'

  • Gold is a very good conductor of electricity. This is why it’s sometimes used in making USB and audio cables

  • Gold is non toxic and can be eaten with food. It is sometimes added to desserts in some expensive restaurants

  • The term Gold originated from an ancient English word 'geolu' which means yellow

  • 24karat gold is the purest gold element

Please read more interesting facts about gold here