Avogadro Number

What is Avogadro Number or Avogadro’s Constant?

The value of Avogadro’s number is approximately 6.022140857(74)×1023 mol−1. The definition of Avogadro’s number is that it tells us the number of particles in 1 mole (or mol) of a substance. These particles could be electrons or molecules or atoms. So, if you wanted to know the number of particles in 3 moles of a substance, the value would be

= 3 x 6.023 x 1023

= 1.81 x 1024 particles

What is so important about Avogadro’s constant?

Simply put, it creates a bridge between chemistry and atomic physics.

In chemistry, we always take a macroscopic approach for measuring stuff. We measure the total volume of a substance, or the temperature, or the mass of a substance and general things along those lines. But if we look at this from the atomic level, knowledge of velocity and momentum of particles are important. The atomic mass is important. Avogadro’s number connects both of these.

How does it do that?

In the atomic level substances are measured as per atomic mass unit. The Atomic mass unit is defined as the 1/12th weight of the mass of one carbon atom. For example, the atomic mass unit of Hydrogen is 1.00794 amu. Now to calculate the ability of a single particle (atom, electron, molecule) to say, carry out a reaction isn’t possible. So instead, chemists came up with a way to link the atomic mass unit and the gram.

1 amu = 1.66 x 10-24 grams

Using this, we are able to convert between measurement in grams and the invisible unit of measurement of the atomic mass unit. And hence, this is the significance of Avogadro’s number.

Try applying your knowledge of Avogadro’s constant to the following question. You have a litre of water are told that the mass of water is 18 amu. So, how many particles are there in this 1 litre of water?

(Hint: 1kg = 1000g)

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Practise This Question

The following figure shows the elliptical path of a planet about the sun. The two shaded parts have equal areas. If t1, t2 be the time taken by the planet to go from A to B and from C to D respectively–