class 9 Mole concept [Atoms and Molecules]

Q. Find the gram molecular mass of water (H2O)
Calculation: 2(H) = 2 x 1 = 2 . 1(O) = 1 x 16 = 16
∴ Gram molecular mass of H2O= 18g

Mole is defined as the amount of substance that contains as many specified elementary particles as the number of atoms in 12g of carbon-12 isotope.

One mole is also defined as the amount of substance which contains Avogadro number (6.023 x 10^23) of particles.

For eg. one mole of oxygen atoms represents 6.023 x 10^23 atoms of oxygen and 5 moles of oxygen atoms contain 5 x 6 . 0 2 3 x 10 ^23 a t oms of oxygen .

Q. Calculate the number of aluminium ions present in 0.051 g of aluminium oxide. [Hint : The mass of an ion is same as that of an atom of the same element.

Sol: 1 mole of Al2O3 contains aluminium ions = 2 × 6.022 × 10^23 = 12.044 × 10^23

Now, 102 g of Al2O3 has number of aluminium ions = 12.044 × 10^23

0.051 g of Al2O3 has number of aluminium ions = (12.044 ×1023 × 0.051)/102= 6.022 × 10^20 ions

Q. How many atoms of oxygen are present in 50g of CaCO3? [Ca = 40, C = 12, O = 16]

Sol: 100 g of CaCO3 contains = 3 × 6.022 × 10^23 atoms of oxygen

50 g CaCO3 will contain = (3 × 6.022 × 10^23 × 50)/100 = 9.033 × 10^23 atoms of oxygen

Q. What is the concentration of hydrogen ions in 1 mol/dm3 of sulphuric acid?

Ans. H2SO4 → 2H+ + SO42–

1 mole of sulphuric acid (H2SO4) contains = 2 g of hydrogen ions.

∴ Concentration of hydrogen ions in 1 mole of sulphuric acid = 2 g/dm3 of hydrogen ions.

Q. What happens to an element 'Z' if its atom gains three electrons?

Ans. It forms Z3– ion. Z + 3e– → Z3–

Q. Calculate the mass of 1 molecule of oxygen. [Given atomic mass of hydrogen = 1 u and oxygen 16 u]

Ans. 6.022 × 10^23 atoms of oxygen weigh = 16 g

1 atom of oxygen will weigh = 16/6.022 ×10^23g = 2.66 × ^10–23 g

Q: (a) Calculate the number of molecules in 8 g of O2.

(b) Calculate the number of moles in 52 grams of He (Helium).

Sol: (a)32 g of O2 contains 6.022 ×10^23 molecules

8 g of O2 contains 6.022 ×10^23 molecules x(8/32)= 1.5055 × 10^23 molecules

(b) Molecular mass of He = 4 g,

4 g of He = 1 mole

52 g of He =(1/4) × 52 = 13 mole

Q. Magnesium and oxygen combine in the ratio of 3 : 2 by mass to form magnesium oxide. How much oxygen is required to react completely with 12 g of magnesium?

Ans. According to the law of constant proportion "In a pure substance same elements are always present in a definite proportion by weight".

3 g of magnesium reacts with oxygen = 2 g

12 g of magnesium reacts with oxygen = 12 × (2/3) = 8 g

Sol: Atomic mass of NH3 = 14 + 3 × 1 = 17

17 g of NH3 atoms = 1 mole

34 gm of NH3 atoms = 34x(1/17) = 2 mole

Q. Calculate the number of moles of the following :

(a) 84 g of nitrogen atom (b) 8.066 × 10^23 number of nitrogen atom (given atomic mass of N = 14)

Ans. (a) 14 g of nitrogen atoms = 1 mole

84 g of nitrogen atom = [1/14] 84 = 6 moles

(b) 6.022 × 10^23 of nitrogen atoms = 1 mole

∴ 8.066 × 10^23 nitrogen atoms = (1/6.022 × 10^23) x 8.066 × 10^23 = 1.339 moles

Q. A sample of vitamin C is known to contain 2.58 × 1024 oxygen atoms. How many moles of oxygen atoms are present in the sample?

Sol: 6.022 × 10^23 of oxygen atoms = 1 mole

Number of oxygen atoms in the sample of vitamin C = 2.58 × 10^24 /6.022 × 10^23 = 4.28 mol in the sample

Q. What is the mass of 0.5 mole of NH3 ? Given Atomic mass of N = 14u, Atomic mass of H = 1u.

Sol: 17 x 0.5 = 8.5g

Q. Calculate the number of particles in 31 g of P4 molecules. Atomic mass of P=31u.

Sol: 1.5x10^23

Q Find the number of moles in 87g of K2SO4 (Atomic mass of K=39u, S=32u, O=16u)

Sol: 0.5 mole

Q. Calculate the number of moles for the following.

(i) 36g of He (At. Mass of He =4 u) (ii) 12.044x10^23 molecules of H2O (At. mass of He=1 u, O= 16 u)