NCERT Solutions For Class 9 Science Chapter 3 Atoms and Molecules

 ● Class 9th NCERT Science Solutions Chapterwise  

Chapter 03. Atoms and Molecules

CHAPTER 03 solutions in english medium                                           Best wishes by Nitish sir 🤞🤞

-------------------------------------------------------------------------------------

NCERT Solutions For Class 9 Science Chapter 3 Atoms and Molecules

Page 27, 28
Question 1. In a reaction 5.3 g of sodium carbonate reacted with 6 g of ethanoic acid. The products were 2.2 g of carbon dioxide, 0.9 g water and 8.2 g of sodium acetate. Show that these observations are in agreement with the law of conservation of mass carbonate.
Answer:

Question 2. Hydrogen and oxygen combine in the ratio of 1 : 8 by mass to form water. What mass of oxygen gas would be required to react completely with 3 g of hydrogen gas?

Question 3. Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass?

Answer: The postulate of Dalton’s atomic theory that is the result of the law of conservation of mass is—the relative number and kinds of atoms are constant in a given compound. Atoms cannot be created nor destroyed in a chemical reaction.

Question 4. Which postulate of Dalton’s atomic theory can explain the law of definite proportions?
Answer: The relative number and kinds of atoms are constant in a given compound.

Page 30
Question 1. Define the atomic mass unit.
Answer: One atomic mass unit is equal to exactly one-twelfth (1/12th) the mass of one atom of carbon-12. The relative atomic masses of all elements have been found with respect to an atom of carbon-12.

Question 2. Why is it not possible to see an atom with naked eyes?
Answer: Atom is too small to be seen with naked eyes. It is measured in nanometres.
1 m = 109 nm

Page 34
Question 1. Write down the formulae of
(i) Sodium oxide
(ii) Aluminium chloride
(iii) Sodium sulphide
(iv) Magnesium hydroxide
Answer: The formulae are

Question 2. What is meant by the term chemical formula?
Answer: The chemical formula of the compound is a symbolic representation of its composition, e.g., chemical formula of sodium chloride is NaCl.

Question 3. How many atoms are present in a
(i) H2S molecule and
(ii) P043- ion?
Answer: 

${{\mathrm{(i)\; PO}}_4}^{3-}$ ion has 4 oxygen atoms and 1 phosphorus atom. So, a total of 5 atoms.

${\mathrm{(ii)\; H}}_{2}S$ molecule has 1 sulphur and 2 hydrogen atoms. So, a total of 3 atoms.

Page 35
Question 1. Calculate the molecular masses of H2, O2, Cl2, C02, CH4, C2H2,NH3, CH3OH.
Answer: The molecular masses are:

Question 2.Calculate the formula unit masses of ZnO, Na2O, K2C03, given atomic masses of Zn = 65 u, Na = 23 u, K = 39 u, C = 12 u, and O = 16 u.
Answer: The formula unit mass of
(i) ZnO = 65 u + 16 u = 81 u
(ii) Na2O = (23 u x 2) + 16 u = 46 u + 16 u = 62 u
(iii) K2C03 = (39 u x 2) + 12 u + 16 u x 3
= 78 u + 12 u + 48 u = 138 u

EXERCISE QUESTIONS PAGE- 36

Question 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.

Answer - SolutionTotal mass of compound = 0.24 g (given)

Mass of boron = 0.096 g (given)
Mass of oxygen = 0.144 g (given)
Percent composition of compond:[ $\frac{Givenmassofelement}{Totalmassofcompound}\times \mathrm{100]\%}$

Thus, percentage of boron by weight in the compound

                                           $=\frac{0.096}{0.24}\times 100\%$   =40%

And, percentage of oxygen by weight in the compound
$=\frac{0.144}{0.24}\times 100\%=60\%$

QUESTION 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?

Answer: 3.0 g of carbon is burnt in 8.00 g of oxygen, and 11.00 g of carbon dioxide is produced. This satisfies the law of definite proportions.

 The calculation for the mass of carbon dioxide:

If 3 g of carbon is burnt in 50 g of oxygen, then 3g of carbon will react only with 8g of oxygen and 42 g of oxygen remains unreacted.
Thus, 11.00 g of carbon dioxide is produced.

Question 3. What are polyatomic ions? Give examples

Answer: Polyatomic ions are ions which consist of more than one atom. For example, nitrate ion, NO₃^-, contains one nitrogen atom and three oxygen atoms.

Question 4. Write the chemical formulae of the following. 
(a) Magnesium chloride
 (b) Calcium oxide 
(c) Copper nitrate 
(d) Aluminium chloride 
(e) Calcium carbonate.

Answer:

Question 5. Give the names of the elements present in the following compounds. 

(a) Quick lime 

(b) Hydrogen bromide 

(c) Baking powder 

(d) Potassium sulphate.

Answer: 

Compound	Chemical Formula	Elements present
Quick lime	CaO	Calcium, Oxygen
Hydrogen bromide	HBr	Hydrogen, Bromine
Baking powder	NaHCO3	Sodium, Hydrogen, Carbon, Oxygen
Potassium sulphate	K2SO4	Potassium, Sulphur, Oxygen

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

(a) Ethyne, C2H2 

(b) Sulphur molecule, S8

(c) Phosphorus molecule, P4 (Atomic mass of phosphorus = 31) 

(d) Hydrochloric acid, HCl 

(e) Nitric acid, HNO3

Answer : (a) Chemical formula of Ethyne C2H2

Molar mass of ethyne = 2 x 12 + 2 x 1

Molar mass of ethyne = 26 g

(b) Chemical formula of Sulphur molecule S8

Molar mass of sulphur molecule = 8 x 32 = 256 g

(c) Chemical formula of Phosphorus molecule, P4 (atomic mass of phosphorus = 31)

Molar mass of phosphorus molecule = 4 x 31 = 124 g

(d) Chemical formula of Hydrochloric acid HCl

Molar mass of hydrochloric acid = 1 + 35.5 = 36.5 g

(e) Chemical formula of Nitric acid HNO3

Molar mass of nitric acid = 1 + 14 + 3 x 16 = 63 g