Atomic Foundation of Matter (Atoms & Molecules): NCERT Solutions (2026)

Given:
Total mass of the compound = $0.24$ g
Mass of boron = $0.096$ g
Mass of oxygen = $0.144$ g

Step-by-step Calculation:

1. Percentage of Boron:
$$ \text{\% of Boron} = \left( \frac{\text{Mass of Boron}}{\text{Total Mass of Compound}} \right) \times 100 $$ $$ \text{\% of Boron} = \left( \frac{0.096}{0.24} \right) \times 100 = 40\% $$

2. Percentage of Oxygen:
$$ \text{\% of Oxygen} = \left( \frac{\text{Mass of Oxygen}}{\text{Total Mass of Compound}} \right) \times 100 $$ $$ \text{\% of Oxygen} = \left( \frac{0.144}{0.24} \right) \times 100 = 60\% $$

Final Answer: The compound contains 40% boron and 60% oxygen by weight.

According to the given data:
$3.0$ g of Carbon reacts completely with $8.0$ g of Oxygen to give $11.0$ g of Carbon Dioxide.

When $3.0$ g of Carbon is burnt in $50.0$ g of Oxygen:
Carbon will still react with only 8.0 g of Oxygen because of its fixed combining ratio. Therefore, the mass of carbon dioxide formed will still be $11.0$ g. The remaining oxygen ($50.0 - 8.0 = 42.0$ g) will remain unreacted.

Law Governing the Answer: This is governed by the Law of Constant Proportions (or Definite Proportions), which states that elements in a compound are always present in a fixed ratio by mass.

A group of atoms bonded together that carry a net electrical charge and act as a single ion is called a polyatomic ion.

Examples:
• Ammonium ion ($NH_4^+$)
• Sulphate ion ($SO_4^{2-}$)
• Carbonate ion ($CO_3^{2-}$)
• Hydroxide ion ($OH^-$)

• (a) Magnesium chloride: Valency of Mg is 2, Cl is 1. Cross-multiplying gives MgCl₂.
• (b) Calcium oxide: Valency of Ca is 2, O is 2. The ratio is 1:1, so the formula is CaO.
• (c) Copper nitrate: Valency of Cu is 2, NO₃ is 1. Formula is Cu(NO₃)₂.
• (d) Aluminium chloride: Valency of Al is 3, Cl is 1. Formula is AlCl₃.
• (e) Calcium carbonate: Valency of Ca is 2, CO₃ is 2. Formula is CaCO₃.

• (a) Quick lime: Its chemical formula is CaO. Elements present: Calcium and Oxygen.
• (b) Hydrogen bromide: Its formula is HBr. Elements present: Hydrogen and Bromine.
• (c) Baking powder (Sodium Bicarbonate): Its formula is NaHCO₃. Elements present: Sodium, Hydrogen, Carbon, and Oxygen.
• (d) Potassium sulphate: Its formula is K₂SO₄. Elements present: Potassium, Sulphur, and Oxygen.

(Given atomic masses: C = 12u, H = 1u, S = 32u, P = 31u, Cl = 35.5u, N = 14u, O = 16u)

• (a) Molar mass of Ethyne ($C_2H_2$): $(2 \times 12) + (2 \times 1) = 24 + 2 =$ $26 \text{ g/mol}$.
• (b) Molar mass of Sulphur ($S_8$): $8 \times 32 =$ $256 \text{ g/mol}$.
• (c) Molar mass of Phosphorus ($P_4$): $4 \times 31 =$ $124 \text{ g/mol}$.
• (d) Molar mass of $HCl$: $1 + 35.5 =$ $36.5 \text{ g/mol}$.
• (e) Molar mass of Nitric acid ($HNO_3$): $1 + 14 + (3 \times 16) = 1 + 14 + 48 =$ $63 \text{ g/mol}$.

Answer: (b) Antoine Lavoisier (Difficulty: Easy)

Answer: (a) 3 and 8 - Atomicity is the number of atoms in a molecule. (Difficulty: Medium)

Answer: (c) 3 (Difficulty: Medium)

Answer: (c) Ag (From its Latin name Argentum). (Difficulty: Easy)

Answer: (b) $CO_2$ (1 Carbon + 2 Oxygen = 3 atoms). (Difficulty: Easy)

Answer: The Law of Constant Proportions states that in a chemical substance, the elements are always present in definite proportions by mass, irrespective of the source or method of preparation of the compound.

Answer: Atomicity is defined as the total number of atoms present in one molecule of an element or a compound.

• Monoatomic: Helium (He), Argon (Ar).
• Diatomic: Oxygen ($O_2$), Nitrogen ($N_2$).

Answer:
Symbol: Al | $SO_4$
Valency: 3 | 2
By cross-multiplying the valencies, the formula is $Al_2(SO_4)_3$. (Difficulty: Medium)

Answer: Atoms are infinitesimally small. The radius of an atom is measured in nanometers ($1 \text{ nm} = 10^{-9} \text{ m}$). Because their size is much smaller than the wavelength of visible light, they cannot be seen with naked eyes or even with ordinary optical microscopes.

Answer:

1. All matter is made of very tiny particles called atoms, which participate in chemical reactions.
2. Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction (supports Law of Conservation of Mass).
3. Atoms of a given element are identical in mass and chemical properties.
4. Atoms of different elements have different masses and chemical properties.
5. Atoms combine in the ratio of small whole numbers to form compounds. (Difficulty: Hard)

(Given atomic masses: H=1u, C=12u, N=14u, O=16u)

Answer:

• (i) $H_2O$: $(2 \times 1) + 16 =$ $18 \text{ u}$
• (ii) $CO_2$: $12 + (2 \times 16) = 12 + 32 =$ $44 \text{ u}$
• (iii) $CH_4$: $12 + (4 \times 1) =$ $16 \text{ u}$
• (iv) $C_2H_6$: $(2 \times 12) + (6 \times 1) = 24 + 6 =$ $30 \text{ u}$
• (v) $NH_3$: $14 + (3 \times 1) =$ $17 \text{ u}$ (Difficulty: Hard)

a) Did the weight of the flask change after the reaction? Why?
(Ans: No, the weight remained the same due to the Law of Conservation of Mass).

b) Write the formula of the white precipitate formed. (Ans: $BaSO_4$).

c) What type of system was used by the teacher?
(Ans: A closed system / sealed flask to prevent any loss of mass). (Difficulty: Medium)

Answer: (A) Both A and R are true, and R is the correct reason for A. By cross-multiplying, we get $Ca(OH)_2$. (Difficulty: Easy)

Answer: (D) A is false, but R is true. Atoms combine in the ratio of small whole numbers, not necessarily 1:1. (Difficulty: Medium)