Atoms and Molecules NCERT Solution, study material, CBSE Notes

NCERT Solution: Atoms and Molecules

Verify by calculating that (b) 240 g of calcium and 240 g magnesium elements have a mole ratio of 3:5.

Ans. Number of moles in 240g Ca metal 240 /40 = 6

Number of moles in 240g of Mg metal 240 10

Ratio 6: 10 = 3: 5

Find the ratio by mass of the combining elements in the following compounds

(a) CaCO₃
Ans. Ca: C: O_³ = 40: 12: 16 × 3 = 40: 12: 48 = 10: 3: 12

(b) MgCl₂
Ans. Mg: CI₂ = 24: 35.55 × 2 = 24: 71

(c) H₂SO₄
Ans. H₂: S: O₄ = 1 × 2: 32: 16 × 4 = 2: 32: 64 = 1: 16: 32

(d) C₂H₅OH
Ans. C₂: H₆: O = 12 × 2: 1 × 6: 16 = 24: 6: 16 = 12: 3: 8

(e) NH₃
Ans. N: H₃ = 14: 1 × 3 = 14: 3

(f) Ca(OH)₂
Ans. Ca: O₂: H₂ = 40: 16 × 2: 1 × 2 = 40: 32: 2 = 20: 16: 1

Calcium chloride when dissolved in water dissociates into its ions according to the following equation.

CaCl₂ (aq) →Ca²⁺ (aq) 2Cl⁻ (aq)
Calculate the number of ions obtained from CaCl₂when 222 g of it is dissolved in water.
Ans. 1 mole of calcium chloride = 111g
∴ 222g of CaCl₂ is equivalent to 2 moles of CaCl₂

Since 1 formula unit CaCl₂ gives 3 ions, therefore, 1 mol of CaCl₂ will give 3 moles of ions
2 moles of CaCl₂ would give 3 ×2 = 6 moles of ions.
No. of ions = No. of moles of ions × Avogadro number
= 6 × 6.022 × 10²³
= 36.132 × 10²³
= 3.6132 × 10²⁴ ions

The difference in the mass of 100 moles each of sodium atoms and sodium ions is 5.48002 g. Compute the mass of an electron.

A sodium atom and ion, differ by one electron. For 100 moles each of sodium atoms and ions there would be a difference of 100 moles of electrons.

Mass of 100 moles of electrons= 5.48002 g

Cinnabar (HgS) is a prominent ore of mercury. How many grams of mercury are present in 225 g of pure HgS? Molar mass of Hg and S are 200.6 g mol^-1 and 32 g mol^-1 respectively.

Molar mass of HgS = 200.6 + 32 = 232.6 g mol^-1

Mass of Hg in 232.6 g of HgS = 200.6 g

Mass of Hg in 225 g of HgS

The mass of one steel screw is 4.11g. Find the mass of one mole of these steel screws. Compare this value with the mass of the Earth (5.98 x 1024kg). Which one of the two is heavier and by how many times?

One mole of screws weigh = 2.475 × 10²⁴ g = 2.475 × 10²¹ kg

Mass of earth is 2.4 ×10³ times the mass of screws

The earth is 2400 times heavier than one mole of screws.

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?

1 mole of oxygen atoms = 6.023 × 10²³ atoms

∴ Number or moles of oxygen atoms

4.28 moles of oxygen atoms.

Raunak took 5 moles of carbon atoms in a container and Krish also took 5 moles of sodium atoms in another container of same weight. (a) Whose container heavier? (b) Whose container has more number of atoms?

(a) Mass of sodium atoms carried by Krish = (5× 23)g =115 g
While mass of carbon atom carried by Raunak = (5× 12)g =60 g
Thus, Krish’s container is heavy
(b) Both the bags have same number of atoms as they have same number of moles of atoms.