**Question 4:** (a) Define and explain mole and Avogadro number with example.

(b) Given the equation CH_{4 (g)} + 2O_{2 (g)} ——→ CO_{2 (g)} + 2H_{2}O_{ (g)} + Heat, how can this be read in terms of particles, moles and masses?

Answer

**Mole in Chemistry**

All substances consist of particles (atoms, molecules, formula units, ions). Mole is the counting unit of particles in a substance. It is defined as “the quantity of a substance which contains 6.022 × 10^{23} particles is called a mole of that substance”.

Number of mole of a substance is usually expressed as n.

It has been found that one gram-atom (or gram-molecule or gram-formula mass) contains 6.022 × 10^{23} particles. Therefore, if mass (in grams) is given, we can calculate the number of moles of particles present in that mass as follow.

Number of moles = (mass in grams)/ (molar mass)

**Avogadro’s number**

As already said, one mole of particles is called Avogadro’s number and it is equal to 6.022 × 10^{23} particles. It is denoted by N_{A}. Therefore,

12 g of Carbon (one gram-atom) contains 6.022 × 10^{23} atoms.

18 g of H_{2}O (one formula-mass) contains 6.22 × 10^{23} molecules of water.

Avogadro’s number is important as it gives the number of particles present in a substance of known mass (in grams).

**Consider an interesting example.**

Fe + S ———-→ FeS

This equation means 1 atom of Iron combine with 1 atom of Sulpher to produce 1 molecule of Ferrous Sulphate.

In other words, it means (1 × 6.22 × 10^{23}) atoms of Fe combine with (1 × 6.022 × 10^{23}) atoms of S to produce (1 × 6.22 × 10^{23}) molecules of FeS.

Since 6.22 × 10^{23} particle is called a mole, therefore, it can also be said as 1 mol iron react with 1 mol of Sulpher to form 1 mol of FeS.

Since 1 mol is one gram-atom of that substance, therefore, equally correct that 56 g of Fe react with 32 g of S to form 88 g of FeS.

This a very useful result because we cannot weigh 1 molecule of Fe or 1 molecule of S for the reaction. However, it is conveniently possible to take to 56 grams of Fe and 32 grams of S to get 88 g of FeS.

Therefore, mole concept has a great importance in stoichiometric calculations.

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