Chemistry

Fundamentals of Mole Concept in 40 minutes by Alakh Pandey Sir


Fundamentals of Mole Concept in 40 minutes by Alakh Pandey Sir

Mole concept is one of the most important topics to start learning high school chemistry. This is a good lecture to understand the fundamentals of mole concept in a short span. Watch the video and make notes from it. It will be of great help. I have tried to make a summary of it below the video.

What is a mole?

The answer is, Mole is a number.
Just like, a dozen is a number and we write, 1 dozen = 12
Similarly, 1 mole = 6.022 × 1023 [CONCEPT : MEMORIZE IT]
So, mole is just a number.

For example, If I say, we have one mole of books, then we can say we have,
1 mole books = 6.022 × 1023 books
Similarly, if we say we have one mole cycles, then
1 mole cycles = 6.022 × 1023 cycles.

So, mole is just a number. and this number is also known as Avogadro’s number.

To understand how to apply the concept, first we will have to learn few fundamentals of Chemistry.

Why the mass of electrons is neglected during the calculation of mass of an atom?

Let us consider a carbon atom. It has 6 protons, 6 neutrons and 6 electrons.
Therefore, 1 C-atom = 6p + 6n + 6e

The protons and the neutrons reside in the nucleus while the electrons revolve around the nucleus in various orbits. 2 electrons in the first orbit and the remaining 4 in the second orbit.

As a result, if we want to find the “mass of 1 atom of carbon”, then we will write,
Mass of 1 C-atom = Mass of 6 Protons + Mass of 6 Neutrons + Mass of 6 electrons.
However, the mass of electrons is negligible compared to the masses of protons and neutrons.
Mass of 1 proton = 1.6726219 × 10-27 kg and
Mass of 1 neutron = 1.674929 x 10-27 kg

As you can see, the mass of proton is nearly equal to the mass of neutron.

On the other hand, the mass of 1 electron = 9.109390 x 10-31 kg
It means the mass of proton is approximately 1837 times larger than that of an electron.

Therefore, the mass of electrons is almost always neglected during calculations of mass of an atom.

What is Atomic Mass Unit (amu)?

The mass of proton or neutron is considered as 1 amu (atomic mass unit).
i.e. 1 amu = 1.67 x 10-27 kg [CONCEPT : MEMORIZE IT]

Consequently,
the mass of 1 C-atom = Mass of a proton x 6 + Mass of a neutron x 6
⇒ the mass of 1 C-atom = (Mass of a proton + Mass of a neutron ) x 6
⇒ the mass of 1 C-atom = (1.67 x 10-27 kg + 1.67 x 10-27 kg ) x 6
⇒ the mass of 1 C-atom = 12 x 1.67 x 10-27 kg
⇒ the mass of 1 C-atom = 12 x 1 amu
⇒ the mass of 1 C-atom = 12 amu

So, atomic mass unit or 1 amu is defined as (1/12)th of the mass of one carbon atom.
In other words, to calculate 1 amu, divide the mass of one carbon atom by 12.

What is Molar Mass or Gram Atomic Mass ?

Certainly, mass of 6.022 × 1023 atoms of C-atom = Mass of 1 C-atom x 6.022 × 1023
⇒ mass of 6.022 × 1023 atoms of C-atom = (12 amu) x 6.022 × 1023
⇒ mass of 6.022 × 1023 atoms of C-atom = 12 x 1.67 x 10-27 kg x 6.022 × 1023
⇒ mass of 6.022 × 1023 atoms of C-atom = 12 x 0.001 kg
⇒ mass of 6.022 × 1023 atoms of C-atom = 0.012 kg
⇒ mass of 6.022 × 1023 atoms of C-atom = 0.012 x 1000 g
⇒ mass of 6.022 × 1023 atoms of C-atom = 12 g

But, 6.022 × 1023 atoms of C-atom = 1 mole of C-atom

⇒ mass of 6.022 × 1023 atoms of C-atom = mass of 1 mole of C-atom
⇒ mass of 1 mole of C-atom = 12 g
This is known as the Molar mass of Carbon or Gram atomic mass

To sum up, [CONCEPT : MEMORIZE IT]
(i) the mass of 1 C-atom = 12 amu
(ii) mass of 6.022 × 1023 atoms of C-atom = 12 g
(iii) mass of 1 mole of C-atom = 12 g

Let us take another example to illustrate the concept,

Mass of 1 oxygen atom = mass of 8 protons + mass of 8 neutrons
⇒ Mass of 1 oxygen atom = (8+8) x 1 amu = 16 amu

Mass of 1 mole of oxygen atom = 16 g

Therefore, we need to just replace the “amu” with “g” while converting from “atomic mass” to “molar mass”. [CONCEPT : MEMORIZE IT]

Illustration 1: Find the number of moles of oxygen atom present in 64 g of oxygen.

Solution :
Since, according to the mole concept, mass of 1 mole of oxygen atom = 16 g

Therefore, applying the concepts of unitary method, we can write,

In 16 g we get 1 mole of oxygen atoms.
So, in 1 g we get 1/16 moles of oxygen atoms.
Therefore, in 64 g we should get (1/16} x 64 moles of oxygen atoms.

i.e. in 64 g we get 4 moles of oxygen atoms.

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Published on August 21st, 2019 | by Abhishek Mandal


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