## 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 × 10**^{23}** ** *[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 × 10^{23} books

Similarly, if we say we have one mole cycles, then

1 mole cycles = 6.022 × 10^{23} 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 × 10^{23} atoms of C-atom = Mass of 1 C-atom x 6.022 × 10^{23}

⇒ mass of 6.022 × 10^{23} atoms of C-atom = (12 amu) x 6.022 × 10^{23}

⇒ mass of 6.022 × 10^{23} atoms of C-atom = 12 x 1.67 x 10^{-27} kg x 6.022 × 10^{23}

⇒ mass of 6.022 × 10^{23} atoms of C-atom = 12 x 0.001 kg

⇒ mass of 6.022 × 10^{23} atoms of C-atom = 0.012 kg

⇒ mass of 6.022 × 10^{23} atoms of C-atom = 0.012 x 1000 g

⇒ mass of 6.022 × 10^{23} atoms of C-atom = 12 g

But, 6.022 × 10^{23} atoms of C-atom = 1 mole of C-atom

⇒ mass of 6.022 × 10^{23} 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*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

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.

*Published on
August 21st, 2019 | by
Abhishek Mandal *