NCERT Solutions for Class 9th Science Chapter 3: Atoms & Molecules

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

Percentage of boron = (0.096 / 0.24) × 100 = 40%
Percentage of oxygen = (0.144 / 0.24) × 100 = 60%

Therefore, the percentage composition of the compound is:
Boron = 40%
Oxygen = 60%

From the data:
3.0 g of carbon + 8.0 g of oxygen   ---->   11.0 g of carbon dioxide

This means 3 g of carbon requires 8 g of oxygen to form 11 g of carbon dioxide.

Now, if 3 g of carbon is burnt in 50 g of oxygen, carbon is the limiting reactant because it can combine only with 8 g of oxygen. The remaining oxygen will be left unused.

So, the amount of carbon dioxide formed will still be 11 g.

Mass of carbon dioxide formed = 11.0 g

Law governing the reaction:
This is governed by the Law of Constant Proportion, which states that a chemical compound always contains the same elements in the same fixed ratio by mass.

Polyatomic ions are ions that contain two or more atoms joined together by covalent bonds and carry a net positive or negative charge. These atoms act as a single charged unit during chemical reactions.

Examples:

• Ammonium ion (NH₄⁺)
• Hydroxide ion (OH⁻)
• Nitrate ion (NO₃⁻)
• Sulphate ion (SO₄²⁻)
• Carbonate ion (CO₃²⁻)

Thus, polyatomic ions are groups of atoms that behave as one charged particle.

(a) Magnesium chloride  ---->  MgCl₂
(b) Calcium oxide  ---->   CaO
(c) Copper nitrate   ----->  Cu(NO₃)₂
(d) Aluminium chloride   ---->  AlCl₃
(e) Calcium carbonate   ------>   CaCO₃

(a) Quick lime (CaO)   ---->   Calcium and Oxygen
(b) Hydrogen bromide (HBr)    ---->   Hydrogen and Bromine
(c) Baking powder (NaHCO₃)    ----->   Sodium, Hydrogen, Carbon, and Oxygen
(d) Potassium sulphate (K₂SO₄)   ------>   Potassium, Sulphur, and Oxygen

(a) Ethyne (C₂H₂)
= (2 × 12) + (2 × 1)
= 24 + 2
= 26 g/mol

(b) Sulphur molecule (S₈)
= 8 × 32
= 256 g/mol

(c) Phosphorus molecule (P₄)
= 4 × 31
= 124 g/mol

(d) Hydrochloric acid (HCl)
= (1 × 1) + (1 × 35.5)
= 1 + 35.5
= 36.5 g/mol

(e) Nitric acid (HNO₃)
= (1 × 1) + (1 × 14) + (3 × 16)
= 1 + 14 + 48
= 63 g/mol

The scientist who laid the foundation of chemical sciences was Antoine Lavoisier.

He is known as the Father of Modern Chemistry because he:

• Established the Law of Conservation of Mass, which states that matter can neither be created nor destroyed in a chemical reaction.
• Gave a clear definition of elements and compounds.
• Helped develop a systematic naming system for chemical substances.

The Law of Conservation of Mass states that mass can neither be created nor destroyed in a chemical reaction.

According to this law, the total mass of the reactants is always equal to the total mass of the products.

Example:
When hydrogen reacts with oxygen to form water,
2H₂ + O₂ → 2H₂O
The total mass of hydrogen and oxygen before the reaction is equal to the mass of water formed after the reaction.

The Law of Constant Proportion states that a chemical compound always contains the same elements in the same fixed ratio by mass, regardless of its source or method of preparation.

Example:
Water (H₂O) always contains hydrogen and oxygen in the ratio of 1:8 by mass, whether it is obtained from a river, rain, or produced in a laboratory.

The organisation that approves the names of elements all over the world is the International Union of Pure and Applied Chemistry (IUPAC).

The symbol of gold is Au

The element used as the reference for atomic mass is Carbon.

The isotope Carbon-12 (C-12) is taken as the standard reference, and its atomic mass is defined as exactly 12 atomic mass units (amu)

The two atoms which can exist independently are Helium (He) and Argon (Ar).

These are noble gases and exist as single atoms because they have a completely filled outermost electron shell, making them stable and unreactive.

The atomic number of magnesium (Mg) is 12.

In a neutral Mg atom, the number of electrons = number of protons = 12.
In a Mg²⁺ ion, the atom loses 2 electrons, so the number of electrons = 12 − 2 = 10.

Therefore:
Mg atom → 12 electrons
Mg²⁺ ion → 10 electrons

Polyatomic ions are ions that consist of two or more atoms bonded together covalently and carry a net positive or negative charge. These atoms act as a single charged unit in chemical reactions.

Examples:

1. Ammonium ion – NH₄⁺
2. Sulphate ion – SO₄²⁻

The atomicity of Argon is 1.

Argon is a noble gas and exists as a single atom (monatomic) because it has a completely filled outermost electron shell, making it stable and unreactive.

Scientists chose 1/16 of the mass of an atom of naturally occurring oxygen as the atomic mass unit because oxygen reacts with many elements to form compounds and is abundantly available in nature, making it a convenient and reliable standard for comparing atomic masses of different elements.

The law of conservation of mass was given by Antoine Lavoisier in 1774.

He stated that mass can neither be created nor destroyed in a chemical reaction. This means that the total mass of the reactants is always equal to the total mass of the products.

Lavoisier proved this law through careful experiments, showing that even when substances change form during a reaction, their total mass remains the same.

The term used to represent the mass of 1 mole molecules of a substance is molar mass. It is expressed in grams per mole (g/mol).

The number 6.023 × 10²³ is called the Avogadro number or Avogadro constant. It represents the number of particles (atoms, molecules, or ions) present in one mole of a substance.

Molecular mass is the sum of the atomic masses of all the atoms present in a molecule. It represents the total mass of one molecule of a substance and is expressed in atomic mass units (amu).

Example:
The molecular mass of water (H₂O) = (2 × 1) + 16 = 18 amu.

The Latin name of  sodium is Natrium.
The Latin name of  mercury is Hydrargyrum.

There are 6.022 × 10²³ atoms in exactly  12 g of carbon.
This number is known as the Avogadro number, and 12 g of carbon represents 1 mole of carbon atoms.

To calculate the formula unit mass of calcium chloride (CaCl₂):

Atomic mass of Ca = 40
Atomic mass of Cl = 35.5

Formula unit mass = 40 + (2 × 35.5) = 40 + 71 = 111 u
So Formula unit mass of CaCl₂  is  111 u

Oxygen (O₂) is a diatomic gas.

In one molecule of H₂SO₄ (sulphuric acid):

Hydrogen (H) = 2 atoms
Sulphur (S) = 1 atom
Oxygen (O) = 4 atoms

Total atoms = 2 + 1 + 4 = 7 atoms
There are 7 atoms in one molecule of H₂SO₄.