Important Solved Questions on Mole Concept

Que 1. Calculate the number of atoms of each type in 5.3 gram of sodium carbonate (Na₂CO₃).\
Ans 1. Given mass of sodium carbonate (Na₂CO₃) = 5.3 g
​Molar mass of sodium carbonate = 23 × 2 + 12 + 16 × 3 = 106g/mol
Number of moles = given mass/ Molar mass = 5.3/106 =   0.05 mol
Now, 1 mole of Na₂CO₃ contains = 2 × 6.022 × 10²³ sodium atoms
∴0.05 of Na₂CO₃ contains = 0.05 × 2 × 6.022 × 10²³ sodium atoms 
= 6.022 × 10²² = 6.022 × 10²² Na atoms.
1 mol of Na₂CO₃ contains  = 6.022 × 10²³ carbon atoms
0.05 mol of Na₂CO₃ contains  = 0.05 × 6.022 × 10²³ carbon atom
∴0.05 of Na₂CO₃ contains  = 0.05 × 3 × 6.022 × 10²³ oxygen atoms 
= 9.03 × 10²³ = 9.03 × 10²³ oxygen atoms.

Que 2. Chlorophyll, the green colouring matter of plants responsible for photosynthesis, contains 2.68% of magnesium by mass. Calculate the number of magnesium atoms in 2.00 g of chlorophyll.
Ans 2. Mass of chlorophyll = 2.0 g
Percentage of Mg = 2.68 g
Mass of Mg in 2.0 g of chlorophyll = 2.68 × 2.0/100 = 0.054 g
6.022 × 10²³ atoms of magnesium = 24 g
24 g of Mg contains = 6.022 × 10²³ atom
0.054 g of Mg contains = 6.022 × 12.0/24 = 1.3 × 10²¹ atoms

Que 3. The mass of 94.5 mL of a gas at S.T.P. is found to be 0.2231 g. Calculate its molecular mass.
Ans 3.

Important Solved Questions on Mole Concept

Que 4. Calculate the number of molecules and number of atoms present in 11.2 liters of oxygen (O₂) at N.T.P.
Ans 4. 1 mole occupies 22.4 litres at STP
Thus 11.2 litres contain = 11.2/22.4 = 0.5 moles
1 mole contains 6.03 ×10²³ = 3.0115 × 10²³ molecules
We know 1 molecule contains 2 atoms
3.0115 × 10²³ molecules contain = 2 × 3.0115 × 10²³ = 6.023 × 10²³ atoms.

Que 5. Calculate the weight of carbon of monoxide having the same number of oxygen atoms as are present in 88 g of carbon dioxide.
Ans 5. Molecular mass of CO₂ = 12 + 2 × 16 = 44
1mol of CO₂ = 44 g
44 g of CO₂ contain = 6.022 × 10²³ molecules
88 g of CO₂ contain = 6.022 × 10²³ × 88/44 = 12.04 × 10²³ molecules
Each molecule of CO₂ contain 2 oxygen atoms
No. of oxygen atoms in 12.04 × 10²³ molecules of CO₂ = 2 × 12.04 × 10²³ = 2.409 × 10²⁴
We have to calculate the mass of CO containing 2.409 × 10²⁴ atoms of oxygen.
Mass of CO containing 6.022 × 10²³ oxygen atoms = 28 g.
Mass of CO containing 2.4092 × 10²³ oxygen atoms = 28/6.022 × 10²³ × 2.409 × 10²⁴ = 112 g

Que 6. Calculate the mass of sodium which contains the same number of atoms as are present in 4 g of calcium (Atomic weight Na = 23, atomic weight Ca = 40).
Ans 6. 40 g of calcium contain atoms = 6.022 × 10²³ atoms
1 g of calcium contains atoms = 6.022 × 10²³/40
15 g of calcium contain atoms = (6.022 × 10²³/40) × 15 = 2.258×10²³
6.022 × 10²³ atoms of sodium have mass = 23 g
1 atom of sodium have mass = 23/6.022 × 10²³
2.258×10²³ atoms of sodium have mass = (23/6.022 × 10²³) × 2.258×10²³
= 8.624 g

Important Solved Questions on Mole Concept

Que 7. How many years it would take to spend Avogadro’s number of rupees at the rate of 10 lakh rupees per second?
Ans 7.

Que 8. How many molecules of aspirin (molar mass = 180 amu) are present in a 50 mg tablets?
Ans 8. We know
Molecules = No of moles × N_A
No. of Moles = Given Mass / Molar Mass = 50/180 = 0.27 mol
Molecules = 0.27 × 6.022 × 10²³
= 1.62 × 10²³ molecules

Que 9. Suppose the chemists had selected 10²⁰ as the number of particles in a mole. The molar mass of oxygen gas would be ( Use Avogadro number = 6.022 × 10²³ ).
Ans 9. 6.022 × 10²³ molecules of oxygen weigh = 32 g
1.0 molecule of oxygen weighs = 32/6.022 × 10²³ = 5.31×10⁻²³g
Mass of 10²⁰ molecules of oxygen = 5.31 × 10⁻²³ × 1020 = 5.31 × 10⁻³g

Que 10. Calculate the number of gold atoms in 300 mg of a gold ring of 20 carat gold (atomic mass of gold = 197, pure gold is 24 carat).
Ans 10.

Important Solved Questions on Mole Concept