How to calculate number of particles| Avogadro’s constant| Mole & Chemical Calculation [Online Video]


Which substance contains the greatest number of atoms in 1 g?
iodine (I2); iron (Fe) ;oxygen (O2)

Syllabus Objectives

At O levels, students must be able to perform calculations involving the number of moles, mass and molar mass of a substance. Also, they must be confident with calculating the number of particles from the number of moles of particles and vice versa.


Particles are so small. There are so many of them it is pretty impossible to count them. Hence, chemist use a unit – mole – to count particles.

One mole of a substance has 6 x 1023 particles. The particles can be atoms, molecules, ions or even electrons, depending on what substance we are referring to.

For example, if we focus on helium, then we are talking about helium atoms.

If we focus on hydrogen gas, then 1 mol of hydrogen gas has 6 x 1023 hydrogen molecules. To determine its number of atoms, we need to multiply the number of molecules by 2. This is because 1 hydrogen molecule has 2 atoms. In 1 mol of hydrogen gas, 6 x 1023 molecules have 2 x 6 x 1023 hydrogen atoms.

Avogadro’s Constant

The value 6 x 1023 is called Avogadro’s constant or Avogadro’s number.

Equal number of moles of substances contain the same number of particles. The reverse is also true.

Formula: number of particles = number of moles x 6 x 1023

Since 1 mole of substance contains 6 x 1023 particles, 2 moles of substance contains 2 x 6 x 1023 particles.0.5 moles of substance contains 0.5 x 6 x 1023 particles.
Hence, we can safely say that, to determine the number of particles, we can use this formula: number of particles = number of moles x 6 x 1023

In this Video

In this video, we will focus on determining the number of atoms in 3 different elements of the same mass. Here, one of the elements, iron, is made of atoms, while the other two, iodine and oxygen, are made of molecules. Hence, for these 2 elements, students should multiply the number of particles (which is molecules) by 2 to determine the number of atoms. This is because in both cases, 1 molecule is made of 2 atoms. To determine the number of atoms in 1 g of the element, students should remember to multiply the value by 2.

Topic: Mole Concept and Chemical Calculations, O Level Chemistry, Singapore

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