Avogadro’s number tells us the number of particles in 1 mole (or mol) of a substance. These particles could be electrons or molecules or atoms. The value of Avogadro’s number is approximately 6.022140857(74)×1023 mol−1.
What is Avogadro’s Number?
The number of units in one mole of any substance is called Avogadro’s number or Avogadro’s constant. It is equal to 6.022140857×1023. The units may be electrons, ions, atoms, or molecules, depending on the character of the reaction and the nature of the substance.
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 the macroscopic world and the microscopic world by relating the amount of substance to the number of particles. It also provides the relationship between other physical constants and properties. Some of them are:
- The relationship between the gas constant R and the Boltzmann constant kB: R=kBNA
- The relationship between the Faraday constant F and the elementary charge e: F=NAe
- The relationship between atomic mass unit u and molar mass constant Mu: 1u=Mu/NA
In chemistry, we always take a macroscopic approach to 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.
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Significance of Avogadro’s Number
- 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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