NCERT Solutions for Class 9 Science Chapter 3 - Atoms and Molecules

Answer:

Mass of sodium carbonate = 5.3 g
Mass of ethanoic acid = 6.0 g
Total mass of the reactants = 5.3 + 6.0
= 11.3 g
Mass of carbon dioxide = 2.2 g
Mass of water = 0.9 g
Mass of sodium ethanoate = 8.2 g
Total mass of the products = 2.2 + 0.9 + 8.2
= 11.3 g
Total mass of the reactants = Total mass of the products Thus, law of conservation of mass is followed.

Answer:

Mass of oxygen gas that reacts with 1 g of hydrogen gas = 8 g
Mass of oxygen gas that reacts with 3 g of oxygen gas = 3 × 8 = 24 g

Answer:

Atoms are indivisible particles and can neither be created nor destroyed during any physical or chemical reaction.

Answer:

Atoms of various elements combine in the simple whole number ratio to form compounds.

Answer:

Atomic mass unit is a mass unit and it is equal to

of mass of an atom of carbon-12.
1 a.m.u. = 1.66 × 10^–27 kg

Answer:

This is because atoms are very–very small. The radius of an atom is of the order of 10^–10 m.

Answer:

Answer:

(i) Aluminium sulphate (ii) Calcium chloride
(iii) Potassium sulphate (iv) Potassium nitrate
(v) Calcium carbonate

Answer:

Chemical formula: The chemical formula of a substance indicates its constituent elements and number of atoms of each combining element present in one molecule of it.

Answer:

(i) An H2S molecule has two atoms of hydrogen and one atom of sulphur.

(ii) A

ion has one atom of phosphorus and three atoms of oxygen.

Answer:

Molecular mass of H₂ = 2 × 1 = 2 u
Molecular mass of O₂ = 2 × 16 = 32 u
Molecular mass of Cl₂ = 2 × 35.5 = 71.0 u
Molecular mass of CO₂ = 1 × 12 + 2 × 16 = 12 + 32 = 44 u
Molecular mass of CH₄ = 12 + 4 × 1 = 12 + 4 = 16 u
Molecular mass of C₂H₆ = 2 × 12 + 6 × 1 = 24 + 6 = 30 u
Molecular mass of C₂H₄ = 2 × 12 + 4 × 1 = 24 + 4 = 28 u
Molecular mass of NH₃ = 14 + 3 × 1 = 14 + 3 = 17 u
Molecular mass of CH₃OH = 12 + 3 × 1 + 16 + 1 = 12 + 3 + 16 + 1 = 32 u

Answer:

Formula unit mass of ZnO = 65 + 16
= 81 u
Formula unit mass of Na₂O = 2 × 23 + 16 = 46 + 16 = 62 u
Formula unit mass of K₂CO₃ = 2 × 39 + 12 + 3 × 16 = 78 + 12 + 48 = 138 u

Answer:

1 mole of carbon has mass = 12 g

\ Mass of 1 atom of carbon =

= 1.99 × 10^–23 g

Answer:

Mass of sodium = 100 g
Atomic mass of sodium = 23 u
$\therefore$ Mass of 1 mole of sodium = 23 g

No. of moles of sodium in 100 g =

$\therefore$ No. of atoms of sodium in 100 g =

=

= 0.262 × 10^–25
= 2.62 × 10^–24
Mass of Fe = 100 g
Atomic mass of Fe = 56 u
$\therefore$ Mass of 1 mole of Fe = 56 g

No. of moles of Fe in 100 g =

=

No. of atoms of Fe in 100 g =

=

= 10.754 × 10^–23
= 1.0754 × 10^–24
$\therefore$ 100 g of sodium has more number of atoms.

Answer:

Weight of compound = 0.24 g
Weight of Boron = 0.096 g
Weight of oxygen = 0.144 g

\ % of B =

% of O =

Answer:

3.00 g of carbon combines with 8.00 g of oxygen to produce 11.00 g of carbon dioxide.
C + O₂ $\to$ CO₂
When 3.00 g of carbon is burnt in 50.00 g of oxygen, 11.00 g of carbon dioxide is produced. This is based upon the law of constant compositions.

Answer:

Polyatomic ion: A group of atoms carrying a charge (+ve or –ve) is called a polyatomic ion.

Examples :

Answer:

Answer:

(a) Quick lime (CaO): The elements present in it are calcium and oxygen.
(b) Hydrogen bromide (HBr): The elements present in it are hydrogen and bromine.
(c) Baking powder (NaHCO₃): The elements present in it are sodium, hydrogen, carbon and oxygen.
(d) Potassium sulphate (K₂SO₄): The elements present in it are potassium, sulphur and oxygen.

Answer:

(a) Molar mass of ethyne, C₂H₂ = 2 × 12 + 2 × 1 = 24 + 2 = 26 g
(b) Molar mass of sulphur molecule, S₈ = 8 × 32 = 256 g
(c) Molar mass of phosphorus molecule, P₄ = 4 × 31 = 124 g
(d) Molar mass of hydrochloric acid, HCl = 1 + 35.5 = 36.5 g
(e) Molar mass of nitric acid, HNO₃ = 1 + 14 + 3 × 16 = 1 + 14 + 48
= 63 g

Answer:

(a) Molar mass of ethyne, C₂H₂ = 2 × 12 + 2 × 1 = 24 + 2 = 26 g
(b) Molar mass of sulphur molecule, S₈ = 8 × 32 = 256 g
(c) Molar mass of phosphorus molecule, P₄ = 4 × 31 = 124 g
(d) Molar mass of hydrochloric acid, HCl = 1 + 35.5 = 36.5 g
(e) Molar mass of nitric acid, HNO₃ = 1 + 14 + 3 × 16 = 1 + 14 + 48
= 63 g

Answer:

(a) Mass of 1 mole of N atoms = 14 g
(b) Mass of 1 mole of Al atoms = 27 g
$\therefore$ Mass of 4 moles of Al atoms = 4 × 27 = 108 g
(c) Mass of 1 mole of sodium sulphite, Na₂SO₃
= 2 × 23 + 32 + 3 × 16
= 46 + 32 + 48 = 126 g
$\therefore$ Mass of 10 moles of Na₂SO₃ = 10 × 126 = 1260 g

Answer:

We know that, the number moles of a substance, n =

(a) No. of moles in 12 g of oxygen gas (O₂) =

(b) No. of moles in 20 g of water (H₂O) =

(c) No. of moles in 22 g of carbon dioxide (CO₂) =

Answer:

(a) 0.2 mole of oxygen atoms:
Mass of 1 mole of oxygen atoms = 16 g
Mass of 0.2 mole of oxygen atoms = 0.2 × 16 = 3.2 g
(b) 0.5 mole of water molecules:
Mass of 1 mole of water molecule (H₂O) = 18 g
$\therefore$ Mass of 0.5 mole of water molecules = 0.5 × 18 = 9 g

Answer:

Mass of solid sulphur (S₈) = 16 g
Mass of 1 mole of sulphur (S₈) = 8 × 32 = 256 g

No. of atoms in 16 g of solid sulphur =

= 3.0115 × 10₂₃

Answer:

(Hint: The mass of an ion is the same as that of an atom of the same element. Atomic mass of Al = 27 u)
Molar mass of aluminium oxide,