Atoms and Molecules | CBSE Class IX, Solutions of NCERT Science Chemistry | NCERT Textbook Solutions

CBSE Science, Class 9, Atoms and Molecules  

NCERT Solutions of Science 
Textbook Chapter Exercise Questions Solved   

Q.1: A 0.24 gm sample of compound of oxygen and boron was found by analysis to contain 0.096 gm of boron and 0.144 gm of oxygen. Calculate the percentage composition of the compound by weight.  

Ans: % of boron in sample = (0.096 ÷ 0.24) × 100 = 40%

        % of oxygen in sample = (0.144 ÷ 0.24) × 100 = 60%

The sample of compound contains 40% boron and 60% oxygen by weight.  

Q.2: When 3.0 gm of carbon s burnt in 8.0 gm oxygen, 11.0 gm of carbon dioxide is produced. What mass of carbon dioxide will be formed when 3.0 gm of carbon is burnt in 50.00 gm of oxygen ? Which law of chemical combination will govern your answer ?   

Ans: When 3.0 gm of carbon s burnt in 8.0 gm oxygen, 11.0 gm of carbon dioxide is produced. It means all of carbon and oxygen are combined in the ratio of 3 : 8 to form carbon dioxide. Thus when there is 3 gm carbon and 50 gm oxygen, then also only 8 gm of oxygen will be used and 11 gm of carbon dioxide will be formed. The remaining oxygen is not used.

This indicates law of definite proportions which says that in compounds, the combining elements are present in definite proportions by mass.     

Q.3: What are polyatomic ions ? Give examples.

Ans: When two or more atoms combine together and behave like one entity with a net charge, then it is called polyatomic ion. For example, oxygen atom and hydrogen atom combine to form hydroxide ion (OH^–). One carbon atom and three hydrogen atom combine to form carbonate ion (CO₃^–2).    

Q.4: Write chemical formulae of the following:

(a) Magnesium chloride (b) Calcium oxide (c) Calcium nitrate (d) Aluminium chloride (e) Calcium carbonate

Ans: (a) MgCl₂ (b) CaO (c) Cu(NO₃)₂ (d) AlCl₃ (e) CaCO₃  

Q.5: Give the names of the elements present in the following compounds:

(a) Quick lime (b) Hydrogen bromide (c) Baking powder (d) Potassium sulphate

Ans: (a) Calcium and oxygen (b) Hydrogen and bromine (c) Sodium, hydrogen, carbon and oxygen (d) Potassium, sulphur and oxygen.

Q.7: What is the mass of:

(a) 1 mole of nitrogen atoms.

(b) 4 moles of aluminium atoms.

(c) 10 moles of sodium sulphite (Na₂SO₃).   

Ans:

(a) 1 mole of nitrogen atoms = 14u = 14gm

(b) 4 moles of aluminium atoms = 4 × 27 = 108u = 108gm

(c) 1mole of sodium sulphite, Na₂SO₃ = 2 × 23 + 1 × 32 + 3 × 16 = 126u = 126gm

      10 moles of sodium sulphite = 126 x 10 = 1260u = 1260gm.  

Q.8: Convert into mole

(a) 12 gm of oxygen gas

(b) 20 gm of water

(c) 22 gm of carbon dioxide    

Ans:

(a) 32 gm of oxygen gas = 1 mole

     12 gm of oxygen gas = 12 ÷ 32 = 0.375 mole.

(b) 18 gm of water = 1 mole

      20 gm of water = 20 ÷ 18 = 1.1 mole.

(c) 44 gm of carbon dioxide = 1 mole

     22 gm carbon dioxide = 22 ÷ 44 = 0.5 mole.  

Q.9: What is the mass of :

(a) 0.2 mole of oxygen atoms

(b) 0.5 mole of water molecules

Ans:

(a) 1 mole of oxygen atoms = 16 gm

    0.2 moles of oxygen atoms = 16 x 0.2 = 3.2 gm

(b) 1 mole of water molecules = 18 gm

     0.5 mole of water molecules = 18 x 0.5 = 9 gm. 

Q.10: Calculate the number of molecules of sulphur (S₈) present in 16 gm of solid sulphur.

 Ans:  1 mole of S₈ = 32 x 8 = 256 gm

          1 mole of S₈ = 6.023 x 10²³ molecules

  So, 256 gm of S₈ = 6.023 x 10²³ S₈ molecules

  Or, 16 gm S₈ = (6.023 x 10²³ ÷ 256) x 16 = 3.76 x 10²² molecules.   

Q.11: Calculate the number of aluminium ions present in 0.51 gm of aluminium oxide.

 Ans: 1 mole of aluminium oxide, Al₂O₃ = 2 x 27 + 3 x 16 = 102u = 102 gm

  That is 102 gm Al₂O₃ has 6.023 x 10²³ Al₂O₃ molecules

 Or, 0.51 gm Al₂O₃ has = (6.023 x 10²³ x 0.51) ÷ 102 = 3.01 x 10²¹ Al₂O₃ molecules.

 We know 1 molecule of Al₂O₃ has 2Al⁺⁺⁺ ions.

Hence, 0.51 gm Al₂O₃ has = 2 x 3.01 x 10²¹ Al⁺⁺⁺ ions        

                                          = 6.023 x 10²¹ aluminium ions.

Further study on this chapter 

=> Class 9, NCERT (CBSE) solutions of Chemistry - Chapter 3, Atoms and Molecules | Answers of Chapter In Text Questions [Read]

=> Class IX, NCERT (CBSE) | Chapter 3, Atoms and Molecules | Study Materials, Notes and Important Sample Questions [Read]

=> Class 9, NCERT (CBSE) Guide | Chapter 3, Atoms and Molecules | Additional Multiple Choice Questions (MCQ) [Read]