Class IX - Atoms and Molecules - Practice for Everyone

Important Questions

1. State the law of constant proportion.

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Ans. It states that, “In a pure chemical substance, the elements are always present in definite proportions by mass”.

Water (H₂O) present in any form and from anywhere remains in fixed proportion i.e., 1:8

2. Give an example to show law of conservation of mass applies to physical changes also.

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Ans. Law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction. However, this law applies to physical changes also.

For example, when ice melts into water, the mass of ice equals to the mass of water, i.e., the mass is conserved. This verifies the law of conservation of mass.

3. Define ‘atomic mass unit’. How is it linked with relative atomic mass?

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Atomic mass unit (u) is 1/12 of the mass of one atom of C-12.

Relative atomic mass of the atom of an element is the average mass of the atom as compared to 1/12^th of the mass of C-12 atom.

4. How do you know the presence of atoms if they do not exist independently for most of the elements?

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Atoms of most of the elements do not exist independently. However, they combine in specific numbers to form molecules or ions which we can feel and know about their presence.

For eg.

A molecule of H₂SO₄ consists of 2 atoms of H + 1 atom of S + 4 atoms of O

A molecule of C₁₂H₂₂O₁₁ consists of 12 atoms of C + 22 atoms of H + 11 atoms of O

Ammonium ion (NH₄⁺) consists of 1 atom of N + 4 atoms of H

5. Mention two postulates of Dalton’s atomic theory that explain:

(a) Law of conservation of mass

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Ans. The elements consists of atoms and that atoms can neither be created nor destroyed” can be used to explain the law of conservation of mass.

Mass (reactants) = mass (products)

(b) Law of constant proportions

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A pure chemical compound always consists of the same elements that are combined together in a fixed (or definite) proportion by mass.

6. Find the ratio of mass of the combining elements in the following compounds:

(a) CaCO₃

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Ans. 40 : 12 : 48 => 10 : 3 : 12

(b) MgCl₂

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Ans. 24 : 71

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Ans. 2 : 32 : 64 =>1 : 16 : 32

7. If the law of constant proportion is true, what weights of these elements will be present in 1.5g of another sample of Calcium Carbonate?

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CaCO₃

Molecular mass = 40 + 12 + 48 = 100g

100 g of CaCO₃ –> 40 g of Ca

1.5 g à 40/100 x 1.5 = 0.6 g

100 g of CaCO₃ –> 12 g of C

1.5 g à 12/100 x 1.5 = 0.18 g

100 g of CaCO₃ –> 48 g of O

1.5 g à 48/100 x 1.5 = 0.72 g

8. Write the chemical symbols of two elements:

(i) Which are formed from the first letter of the elements’ name?

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C, O, H, etc

(ii) Whose names have been taken from the names of the elements in Latin?

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Fe, Au, Ag, etc

(iii) Which of are formed from the first two letters of the elements’ name?

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Ar, Ne, He, etc

9. An element has valency 3. Write the formula of its oxide.

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E O

3 2

= E₂O₃

10. What are polyatomic ions? Give two examples.

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Ans. A group of atoms having a charge is known as polyatomic ion.

Eg. NH₄⁺, SO₄^2-

11. Give one example each of

(i) Monovalent cation – Na⁺, Li⁺, K⁺, NH₄⁺

(ii) Bivalent cation – Mg²⁺, Ca²⁺

(iii) Monovalent anion – OH^–, H^–, Cl^–

(iv) Bivalent anion – O^2-, S^2-

12. What is meant by a chemical formula? Give Examples.

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Ans. A chemical formula of a compound shows its constituent elements and the number of atoms of each combining element.

Eg. Chemical formula of ammonia is NH₃, Water is H₂O, Carbon dioxide is CO₂

13. Write down the formula of:

(i) Sodium oxide

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Na O

1+ 2-

=Na₂O

(ii) Aluminium oxide

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Al O

3+ 2-

Al₂O₃

(iii) Sodium sulphide

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Na S

1+ 2-

Na₂S

(iv) Magnesium hydroxide

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Mg (OH)

2+ 1-

Mg(OH)₂

14. State two examples in each case and write their chemical formulae:

(a) Molecules having one kind of atoms only

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Ans. H₂, O₂, N₂

(b) Molecules having two different kinds of atoms

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Ans. H₂O, CO₂, HCl

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Ans. CaCO₃, H₂SO₄, NaOH

15. Write the formula and the names of the ions formed by the combination of

(i) Fe³⁺ and SO₄^2-

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Fe SO₄

3+ 2-

Fe₂(SO₄)₃ – Iron (III) sulphate

(ii) NH₄⁺ and CO₃^2-

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NH₄ CO₃

1+ 2-

(NH₄)₂CO₃– Ammonium Carbonate

16. What is an ion? Give one difference between cation and anion.

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Ans. Charged atom is called as an ion. The ion can be positively charge called cation or negatively charged called anion.

Eg. Cation – Na⁺

Anion – Cl^–

17. Define atomicity. Write the atomicity of the following molecules:

(i) Sulphur

(ii) Phosphorus

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Ans. It is the number of atoms present in one molecule of a substance

(i) 8

(ii) 4

18. What are ionic and molecular compounds? Give examples.

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Ans

Ionic compound	Molecular compound
Made up of combination of opposite ions	Made up of atoms by sharing of electrons
Eg. Na⁺ Cl^–	Eg. H₂O
Due to transfer of electrons	Exists as neutral substance

19. Write down the names of compounds represented by the following formulae:

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(i) Al₂(SO₄)₃ – Aluminium Sulphate

(ii) CaCl₂ – Calcium Chloride

(iii) K₂SO₄ – Potassium Sulphate

(iv) KNO₃ – Potassium Nitrate

(v) CaCO₃ – Calcium Carbonate

20. How many atoms are present in

(i) H₂S molecule – 3 atoms

(ii) PO₄^3- – 5 atoms

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21. Give the names of the elements present in the following compounds

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(a) Quick lime – CaO – Calcium and Oxygen

(b) Hydrogen Bromide – HBr – Hydrogen and Bromine

(d) Potassium sulphate – K₂SO₄ – Sulphar, Oxygen, Potassium

22. What is molar mass? What are its units?

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Ans. Mass of 1 mole of a substance. Unit – g/mol

23. Distinguish between molecular mass and molar mass.

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Ans.

Molecular Mass	Molar Mass
The molecular mass of a substance is the sum of the atomic masses of all atoms in a molecule	The mass of 1 mole of any substance is called is its molar mass.
H₂O = 1×2 + 16 = 18u	Eg. 18g/mol

24. Define one mole, illustrate its relationship with Avogadro constant.

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Ans. One mole of any species (atoms, molecules, ions or particles) is that quantity in number having mass equal to its atomic or molecular mass in grams.

The number of particles (atoms, molecules or ions) present in 1 mole of any substance is fixed, with a value of 6.022 x 10²³. This number is called Avogadro constant or Avogadro number.

25. What is the mass of:

(a) 0.2 mole of oxygen atoms

(b) 0.5 mole of water molecules

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(a) Number of moles (n) = G.M/M.M

M.M of oxygen = 16g

n = 0.2

G.M = n x M.M

= 0.2 x 16

= 3.2 g

(b) M.M of H2O = 2 x 1 + 1 x 16 = 18g

n = 0.5

G.M = n x M.M

= 0.5 x 18

= 9 g

26. Calculate the number of molecules of SO₂ present in 44 g of it.

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M.M of SO₂ = 32 + 2 x 16 = 64g

Number of molecules = G.M/M.M x Avogadro constant

= 44/64 x 6.023 x 10²³

= (3.011 x 11 x 10²³)/8

= 3.76 x 10²³

27. If one mole of carbon atoms weights 12 grams, what is the mass (in grams) of 1 atom of carbon?

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6.022 x 10²³C-atoms –>12 g of C

1 atom –> 12/6.022 x 10²³

1 atom of C –>1.9 x 10^-23 g

28. Calculate the molar mass of the following substances

(a) Ethyne, C₂H₂

(b) Sulphur molecule, S₈

(d) Hydrochloric acid, HCl

(e) Nitric acid, HNO₃

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Ans. (a) 2 x 12 + 2 x 1 = 24 + 2 = 26 g/mol

(b) 8 x 32 = 256 g/mol

(d) 1 x 1 + 35.5 = 36.5 g/mol

(e) 1 x 1 + 1 x 14 + 2 x 16 = 1 + 14 + 32 = 47 g/mol

29. What is the mass of:

(a) 1 mole of nitrogen atoms

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M.M of nitrogen = 14g

n = 1 mole

G.M = n x M.M

=1 x 14

= 14 g/mol

(b) 4 moles of aluminium atoms (Atomic mass of aluminium = 27)

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M.M of Al = 27

n = 4 moles

G.M = n x M.M

= 4 x 27

= 108g/mol

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M.M of sodium sulphite = 2 x 23 + 32 + 3 x 16 = 126 g/mol

n = 10 mole

G.M = n x M.M

= 10 x 126

= 1260 g/mol

29. How many gram molecules of H₂SO₄ are present in 4.9 g of the acid?

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1 mole –>1 g atoms

1 g molecules
1 g ions

M.M = 2 x 1 + 32 + 4 x 16 = 98 g/mol

G.M = 4.9 g

n = G.M/M.M

= 4.9/98 = 0.05 g molecules

30. Convert into mole

(a) 12 g of oxygen gas (O₂)

(b) 20 gm of water (H₂O)

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(a) M.M of oxygen gas = 2 x 16 = 32g

G.M = 12 g

n = G.M/M.M = 12/32 moles = 0.375 moles

(b) M.M of water = 2 x 1 + 16 = 18g

G.M = 20g

n = G.M/M.M = 20/18 moles = 1.11 moles

G.M = 22 g

n = 22/44 moles = 0.5 moles

Class IX – Atoms and Molecules