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

Atoms and Molecules
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NCERT Class 9 Science chapter 3 has good weightage in the examination. Every year minimum 3-4 questions are being asked from this chapter in almost all school examinations. In chapter 3, you will learn about Atoms and Molecules. This chapter includes Mole Concept also which you will study in class 11 also. So, this chapter builds the foundation of the Mole concept for you. It’s very important for you to clearly understand the basics of Mole Concept to solve all the numerical problems based on this topic. You will also get to know laws of chemical combination, symbols of elements, etc. On Swiflearn, you will get all solved questions of NCERT by experts. Also, there are many diagrams throughout the chapter that will help you to understand the concepts better. You can go through the text and then check out the solved questions to get a vivid idea of how the questions are framed in the examination.

NCERT Solutions for Class 9 Science Chapter 3 includes Intext Exercise 1, 2, 3, 4, 5, and an NCERT exercise. It contains a total of 11 questions in its NCERT exercise. Chapter 3 of Class 9 Science includes application and memory-based questions. You can practice these questions to test and enhance your application skills and cognizance.

Ncert solutions

NCERT Solutions for Class 9 Science Chapter 3 : Atoms and Molecules Ex 3.1

Exercise 3.1

Question 1.
In a reaction, 5.3 g of sodium carbonate reacted with 6 g of acetic 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.
Sodium carbonate + acetic acid→ sodium acetate + carbon dioxide + water

Solution: Refer pdf.

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?

Solution: Refer pdf.

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

Solution:
Atoms are indivisible particles. It cannot be created or destroyed in a chemical reaction.

Question 4:
Which postulate of Dalton’s atomic theory can explain the law of definite
proportions?

Solution:
The relative number and kinds of atoms are constant in a given compound.

Exercise 3.2

Question 1.
1. Define the atomic mass unit.

Solution:
One atomic mass unit is a mass unit equal to exactly one-twelfth (1/12th) the mass of one atom of carbon-12

Question 2.
Why is it not possible to see an atom with naked eyes?

Solution
The size of the atom is too small to be visible with naked eyes.

Exercise 3.3

Question 1.
Write down the formulae of
(i) Sodium oxide (ii) aluminium chloride (iii) sodium suphide (iv) magnesium
hydroxide

Solution: Refer pdf.

Question 2.
Write down the names of compounds represented by following formulae:
(i) Al2(SO4)3 (ii) CaCl2 (iii) K2SO4 (iv) KNO3 (v) CaCO3.

Solution: Refer pdf.

Question 3.
What is meant by the term chemical formula?

Solution:
The chemical formula of a compound is a symbolic representation of its composition.

Question 4.
How many atoms are present in a (i) H2S molecule and (ii) PO4
3– ion?

Solution: Refer pdf.

Exercise 3.4

Question 1.
Calculate the molecular masses of H2, O2, Cl2, CO2, CH4, C2H6, C2H4, NH3,
CH3OH.

Solution: Refer pdf.

Question 2.
Calculate the formula unit masses of ZnO, Na2O, K2CO3, given atomic masses of Zn = 65 u, Na = 23 u, K = 39 u, C = 12 u, and O = 16 u.

Solution: Refer pdf.

Question1.
If one mole of carbon atoms weighs 12 grams, what is the mass (in grams) of 1 atom of carbon?

Solution: Refer pdf.

Question 2.
Which has more number of atoms, 100 grams of sodium or 100 grams of iron (given, atomic mass of Na = 23 u, Fe = 56 u)?

Solution: Refer pdf.

Exercise 3.6

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.

Solution: Refer pdf.

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?

Solution: As it is given that only 8 g oxygen is required to react with 3 g carbon, 42 g of oxygen will remain unused. This is governed by law of constant proportions.

Question 3.
What are polyatomic ions? Give examples.

Solution: Refer pdf.

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

Solution: Refer pdf.

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

Solution:  Refer pdf.

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

Solution:  Refer pdf.

Question 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 (Na2SO3)?

Solution:  Refer pdf.

Question 8.
Convert into mole. (a) 12 g of oxygen gas (b) 20 g of water (c) 22 g of carbon
dioxide.

Solution:  Refer pdf.

Question 9.
What is the mass of: (a) 0.2 mole of oxygen atoms? (b) 0.5 mole of water
molecules?

Solution:  Refer pdf.

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

Solution:  Refer pdf.

Question 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)

Solution:  Refer pdf.

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