Solved Examples on Mole Concept

Example 1: Calculate the molecular mass of Ammonium Sulphate (NH₄)₂SO₄. 

Solution:

Since, the relative atomic masses of N = 14, H = 1, S = 32, O =16

Therefore, the molecular mass of the given compound is,

= 2 (14 × 1 + 1 × 4) + 32 + 16 × 4

= 2(14 + 4) + 32 + 64

= 2 × 18 + 32 + 64

= 36 + 32 + 64

= 132 amu

Example 2: The molecular mass of H₂SO₄ is 98 a.m.u. What does it mean?

Solution:

It means that one molecule of H₂SO₄ is 98 times as heavy as 1/12 the mass of a C-12 atom.

Example 3: Calculate the number of atoms present in 18g of H₂O.

Solution:

The 18 g of H₂O ⇒ 1 mole of H₂O ⇒ N_A molecules

Now, 1 molecule of H₂O contain 3 atoms

⇒ 1 mole H₂O will contain 3*N_A atoms = 3 × 6.022 × 10²³ atoms = 1.8066 × 10²⁴ atoms

Example 4: How many moles are present in 200g of NaOH?

Solution:

The mass of 1 mole of NaOH = 23 + 16 + 1= 40 g

Therefore, in 200g of NaOH the number of moles present = 200g / 40g mol^-1 = 5 mol

Example 5: Calculate the mass of an atom of oxygen element.

Solution:

Mass of 1 mole of oxygen = 16g

No. of atoms in 1 mole of oxygen = N_A

Therefore, mass of one atom of oxygen = 16g / N_A = 16 / (6.022 × 10²³) = 2.657 × 10^-23 g

Example 6: Find the ratio of moles of oxygen atoms present in the compounds H₂SO₄, H₂SO₃, and SO_2.

Solution:

1 mole of H₂SO₄ contains 4 × N_A atoms of oxygen

1 mole of H₂SO₃ contains 3 × N_A atoms of oxygen

1 mole SO₂ contains 2 × N_A atoms of oxygen 

Therefore the required ratio is 4 × N_A : 3 × N_A : 2 × N_A = 4 : 3 : 2

Example 7: How many moles of hydrogen and oxygen gas are required to produce 13 moles of water?

Solution:

Chemical equation of water formation:

H₂ + O₂ ⇢ H₂O

Now the balanced equation is

2H₂ + O₂ ⇢ 2H₂O

Thus we can deduce that 2 moles of hydrogen gas and 1 mole of oxygen gas combine together to form 2 moles of water. We can write the equation as :

H₂ (1 mole) + O₂ (1/2 mole)⇢ H₂O (1 mole)        

Hence, for production of 13 moles of water the required chemical equation would be:

H₂ (13 moles) + O₂ (13/2 moles) ⇢ H₂O (13 moles)

Hence, for production of 13 moles of water we need 13 moles of hydrogen and 6.5 moles of oxygen gas.

Example 8: Calculate moles of electrons present in 104 g of acetylene gas.

Solution:

Now, formula for acetylene gas is C₂H₂. It’s structure is H—C≡C—H. Therefore, number of electrons present in 1 molecule of acetylene are 14.

Now, mass of 1 mole of acetylene is 26g ⇒ 104g of acetylene are 4 moles.

Now, 1 mole of acetylene = N_A molecules

⇒ N_A molecules have 14 × N_A electrons

⇒ 4 moles of acetylene have 4 × 14 × N_A electrons = 56 × N_A electrons = 56 moles of electrons.

Mole concept is the method used to express the amount of substance. This has been experimentally proving that one gram atom of any element, as well as one gram molecule of any substance, contains the same amount of entities. The experimentally decided number is found to be 6.022137 × 10²³. After the discovery of the mole concept, the problem of finding absolute atomic masses of atoms was solved. It was so because the mole concept helps to count the number of atoms or molecules in a definite amount of the given substance. Let’s learn more about mole concept formulas and examples.