Question 4: (a) Define and explain mole and Avogadro number with example.
(b) Given the equation CH4 (g) + 2O2 (g) ——→ CO2 (g) + 2H2O (g) + Heat, how can this be read in terms of particles, moles and masses?
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 × 1023 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 × 1023 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)
As already said, one mole of particles is called Avogadro’s number and it is equal to 6.022 × 1023 particles. It is denoted by NA. Therefore,
12 g of Carbon (one gram-atom) contains 6.022 × 1023 atoms.
18 g of H2O (one formula-mass) contains 6.22 × 1023 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 × 1023) atoms of Fe combine with (1 × 6.022 × 1023) atoms of S to produce (1 × 6.22 × 1023) molecules of FeS.
Since 6.22 × 1023 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.