<< Chapter < Page Chapter >> Page >

This result is so important that scientists decided to use a special unit of measurement to define this quantity: the mole or 'mol'. A mole is defined as being an amount of a substance which contains the same number of particles as there are atoms in 12 g of carbon. In the examples that were used earlier, 24.31 g magnesium is one mole of magnesium, while 40.08 g of calcium is one mole of calcium. A mole of any substance always contains the same number of particles.


The mole (abbreviation 'n') is the SI (Standard International) unit for 'amount of substance'. It is defined as an amount of substance that contains the same number of particles (atoms, molecules or other particle units) as there are atoms in 12 g carbon.

In one mole of any substance, there are 6.023 x 10 23 particles. This is known as Avogadro's number .

Avogadro's number

The number of particles in a mole, equal to 6.023 x 10 23 . It is also sometimes referred to as the number of atoms in 12 g of carbon-12.

Interesting fact

The original hypothesis that was proposed by Amadeo Avogadro was that 'equal volumes of gases, at the same temperature and pressure, contain the same number of molecules' . His ideas were not accepted by the scientific community and it was only four years after his death, that his original hypothesis was accepted and that it became known as 'Avogadro's Law'. In honour of his contribution to science, the number of particles in one mole was named Avogadro's number .

Moles and mass

  1. Complete the following table:
    Element Relative atomic mass (u) Sample mass (g) Number of moles in the sample
    Hydrogen 1.01 1.01
    Magnesium 24.31 24.31
    Carbon 12.01 24.02
    Chlorine 35.45 70.9
    Nitrogen 42.08
  2. How many atoms are there in...
    1. 1 mole of a substance
    2. 2 moles of calcium
    3. 5 moles of phosphorus
    4. 24.31 g of magnesium
    5. 24.02 g of carbon

Molar mass

Molar mass

Molar mass (M) is the mass of 1 mole of a chemical substance. The unit for molar mass is grams per mole or g.mol - 1 .

Refer to [link] . You will remember that when the mass, in grams, of an element is equal to its relative atomic mass, the sample contains one mole of that element. This mass is called the molar mass of that element.

It is worth remembering the following: On the Periodic Table, the relative atomic mass that is shown can be interpreted in two ways.

  1. The mass of a single, average atom of that element relative to the mass of an atom of carbon.
  2. The mass of one mole of the element . This second use is the molar mass of the element.
The relationship between relative atomic mass, molar mass and the mass of one mole for a number of elements.
Element Relative atomic mass (u) Molar mass (g.mol - 1 ) Mass of one mole of the element (g)
Magnesium 24.31 24.31 24.31
Lithium 6.94 6.94 6.94
Oxygen 16 16 16
Nitrogen 14.01 14.01 14.01
Iron 55.85 55.85 55.85

Calculate the number of moles of iron (Fe) in a 111.7 g sample.

  1. If we look at the periodic table, we see that the molar mass of iron is 55.85 g.mol - 1 . This means that 1 mole of iron will have a mass of 55.85 g.

  2. If 1 mole of iron has a mass of 55.85 g, then: the number of moles of iron in 111.7 g must be:

    111 . 7 g 55 . 85 g . m o l - 1 = 2 m o l

    There are 2 moles of iron in the sample.

Got questions? Get instant answers now!

Questions & Answers

during a snooker competition ,a 200g ball A m moving with velocity va collide head on with a identical ball B that was at rest.A after the collision ball A remains at rest wile ball B moves on with a velocity of 4m/s? With what speed was ball a moving before the collision
mathew Reply
a vector can be resolved into a horizontal component only
Lizoh Reply
how to calculate normal force
Chauke Reply
how to calculate wavelength
Hello. How does a real gas behave under low temperature and high pressure?
Valerie Reply
does a vector quantity include force and distance?
Lebo Reply
what's the difference between a vector and a scalar?
vector is the physical quantity with magnitude and direction Scalar is the Physical quantity with magnitude only
Newton's second law of motion
Thelma Reply
Newton second law motin
Newton's second law of motion: When a resultant/net force acts on an object, the object will accelerate in the direction of the force at an acceleration directly proportional to the force and inversely proportional to the mass of the object.
newtons third law of motion
what is tail to tail method
Thobile Reply
a person was standing in a stationary lift / elevator and scale shown is 490N and then 470N when the lift started to move. Did the lift go up or down?
Kamva Reply
How do you state snell's law
Mlondi Reply
The ratio of the sine of the angle of incidence in one medium to the sine of the angle of refraction in the other medium is constant
who was the first person to discover nuclears bomb
Ismael Reply
how to calculate resistance in grade 11
Ngomane Reply
last year memo in 2018 June exam
can I have last june question paper2
is there any physics e-copy textbooks?
states the Newton's second law of motion
Shallin Reply
In words it says:" when a net force is applied to an object of mass, It accelerates in the direction of the net force. The acceleration is directly proportional to the net force and inversely proportional to the mass".
And in symbols: Fnet = ma
thank you
hello everyone
which degree is usually be a refractive angle before it complete a totally reflection?
yes obviously 90 degree
how does hydrogen bonds differ from London force
Madzivha Reply
Hydrogen bonds are the strongest intermolecular forces and London forces are the weakest. Hydrogen bonds exist between polar molecules ( they are a special case of dipole-dipole forces ), while London forces occur between non-polar molecules.
how come the resultant force is 0
Andrew Reply
It's when you have equivalent forces going different directions then your resultant will be equal to zero
describe what john's experiment proves about water molecules?
Fanozi Reply

Get the best Siyavula textbooks: gr... course in your pocket!

Source:  OpenStax, Siyavula textbooks: grade 11 physical science. OpenStax CNX. Jul 29, 2011 Download for free at http://cnx.org/content/col11241/1.2
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Siyavula textbooks: grade 11 physical science' conversation and receive update notifications?