# NCERT Solutions for Class 9 Science Atoms and Molecules part 2

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NCERT solutions for Science Atoms and Molecules

## NCERT Class 9 Science Chapter wise Solutions

• 01 –  Matter in Our Surroundings
• 02 – Is Mattter Around us Pure
• 03 – Atoms and Molecules
• 04 – Structure of the Atom
• 05 – The Fundamental Unit of Life
• 06 – Tissues
• 07 – Diversity in Living Organisms
• 08 – Motion
• 09 – Force and Laws of Motion
• 10 – Gravitation
• 11 – Word and Energy
• 12 – Sound
• 13 – Why Do We Fall Ill
• 14 – Natural Resources
• 15 – Improvement in Food Resources

## NCERT Solutions for Class 9 Science Atoms and Molecules part 2

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

Ans. Mass of the given sample compound = 0.24g

Mass of boron in the given sample compound = 0.096g

Mass of oxygen in the given sample compound = 0.144g

% composition of compound = % of boron and % of oxygen

Therefore % of boron = mass of boron 100/mass of the sample compound

= 40%

Therefore % of oxygen = mass of oxygen 100/mass of the sample compound

= 60%

NCERT Solutions for Class 9 Science Atoms and Molecules part 2

###### 2.When 3.0 g of carbon is burnt in 8.00 g oxygen, 11.00 g of carbon dioxide is produced. What mass of carbon dioxide will be formed when 3.00 g of carbon is burnt in 50.00 g of oxygen? Which law of chemical combination will govern your answer?

Ans. According to the law of chemical combination of constant proportions “in a chemical compound the elementary constituents always combine in constant proportions by weight/mass”. Therefore, whether 3 g carbon is burnt in 8 g oxygen or 3g carbon is burnt in 50g oxygen in both cases only 11g carbon dioxide will be formed.

###### 3.What are polyatomic ions? Give examples.

Ans. When two or more atoms in a group is having a charge, such is called a polyatomic ion. For etc.

NCERT Solutions for Class 9 Science Atoms and Molecules part 2

###### 4.Write the chemical formulae of the following.

(a) Magnesium chloride

(b) Calcium oxide

(c) Copper nitrate

(d) Aluminium chloride

(e) Calcium carbonate.

Ans.

###### 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.

NCERT Solutions for Class 9 Science Atoms and Molecules part 2

###### 6. Calculate the molar mass of the following substances.

(a) Ethyne,

(b) Sulphur molecule,

(c) Phosphorus molecule, (Atomic mass of phosphorus= 31)

(d) Hydrochloric acid, HCl

(e) Nitric acid,

Ans. (a)Ethyene = =24 + 2= 26 u =26 g

(b)Sulphur molecular = = 256 u= 256 g

(c)Phosphorus molecule = = 124 u= 124 g

(d)Hydrochloric acid = HCl = 1+ 35.5 = 36.5 u= 36.5 g

(e)Nitric acid = = 1 + 14 + = 15 + 48 = 63 u =63 g

NCERT Solutions for Class 9 Science Atoms and Molecules part 2

###### 7.What is the mass of—

(a) 1 mole of nitrogen atoms?

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

(c) 10 moles of sodium sulphite?

Ans. (a)Atomic mass of nitrogen is 14 u.

therefore 1 mol of N = 14g

(b)Atomic mass of aluminium = 27u

therefore 1 mol of Al = 27g and so 4 mol of Al

(c)molecular mass of = 46 + 32 + 48 = 126 u

therefore 1 mol of has weight/mass 126g.

hence, 10 mol of = 1260g

###### 8.Convert into mole.

(a) 12 g of oxygen gas

(b) 20 g of water

(c) 22 g of carbon dioxide.

Ans. (a)molecular mass of = 32 u= 32g(1 mole)

since 32 g of =1mole then 12g of =0.375mole.

(b)molecular mass of + 16 = 18 u= 18g(1mole)

20g = 1.11mole.

(c)molecular mass of = 12 + 32 = 44 u= 44g (1mole)

22g of = 0.5mole.

NCERT Solutions for Class 9 Science Atoms and Molecules part 2

###### 9.What is the mass of:

(a) 0.2 mole of oxygen atoms?

(b) 0.5 mole of water molecules?

Ans. (a) since 1 mole of O = atomic mass of O = 16u=16g

then 0.2mole of

(b)1mol of H2O = molecular mass of =18 u =18g

then 0.5mol of

###### 10.Calculate the number of molecules of sulphur (S8) present in 16 g of solid sulphur.

Ans. 1mol of = molecular mass of = 256u=256g

since 256g of atoms (Avogadro number)

16g of molecules

NCERT Solutions for Class 9 Science Atoms and Molecules part 2

###### 11.Calculate the number of aluminium ions present in 0.051 g of aluminium oxide.

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

Ans. 1mol of = molecular mass of = 102u=102g

aluminium ions present in

102 g of contains aluminium ions

then 0.051 g contains aluminium ions

## NCERT Solutions for Class 9 Science

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