NCERT Class 9 Chapter 3 – Atoms and Molecules
This chapter deals with the introduction of atoms and molecules that constitute matter. The important topics touched upon have been summarized as follows: Introduction to Atoms and Molecules: Explanation of the basic units of matter and their interaction. Structure of Atoms: Studying the atomic structure – protons, neutrons and electrons. Composition of molecules : How atoms are combined into molecules and also studying molecular formulae. Chemical Reaction: Reactions involving atoms and molecules and laws governing these reactions.
Access Answers to NCERT Class 9 Chapter 3 – Atoms and Molecules
Students can access the NCERT Class 9 Chapter 3 – Atoms and Molecules. Curated by experts according to the CBSE syllabus for 2023–2024, these step-by-step solutions make Science much easier to understand and learn for the students. These solutions can be used in practice by students to attain skills in solving problems, reinforce important learning objectives, and be well-prepared for tests.
Exercise-3.1
In a reaction, 5.3g of sodium carbonate reacted with 6 g of acetic acid. The products were 2.2 g of carbon dioxide, 0.9 g of 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
Sodium carbonate + acetic acid → Sodium acetate + carbon dioxide + water
5.3g 6g 8.2g 2.2g 0.9g
As per the law of conservation of mass, the total mass of reactants must be equal to the total mass of
products.
As per the above reaction, L.H.S. = R.H.S. i.e., 5.3g + 6g = 2.2g + 0.9 g + 8.2 g = 11.3 g
Hence, the observations are in agreement with the law of conservation of mass.
Hydrogen and oxygen combine in a 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?
We know hydrogen and water mix in a ratio 1: 8.
For every 1g of hydrogen, it is 8g of oxygen.
Therefore, for 3g of hydrogen, the quantity of oxygen = 3 x 8 = 24g
Hence, 24g of oxygen would be required for the complete reaction with 3g of hydrogen gas.
Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass?
The relative number and types of atoms are constant in a given composition, says Dalton’s atomic theory, which is based on the rule of conservation of mass.
“Atoms cannot be created nor be destroyed in a chemical reaction.”
Which postulate of Dalton’s atomic theory can explain the law of definite proportions?
The postulate of Dalton’s atomic theory that can explain the law of definite proportions is that the
relative number and kinds of atoms are equal in given compounds.
Exercise-3.2
Define the atomic mass unit.
An atomic mass unit is a unit of mass used to express the weights of atoms and molecules where one
atomic mass is equal to 1/12th the mass of one carbon-12 atom.
Why is it not possible to see an atom with the naked eyes?
Firstly, atoms are minuscule in nature, measured in nanometers. Secondly, except for atoms of noble
gases, they do not exist independently. Hence, an atom cannot be visible to the naked eyes.
Exercise-3.3
Write down the formulae of
(i) sodium oxide
(ii) aluminium chloride
(iii) sodium sulphide
(iv) magnesium hydroxide
The following are the formulae:
(i) sodium oxide – Na2O
(ii) aluminium chloride – AlCl3
(iii) sodium sulphide – Na2S
(iv) magnesium hydroxide – Mg (OH)2
Write down the names of compounds represented by the following formulae:
(i) Al2(SO4)3
(ii) CaCl2
(iii) K2SO4
(iv) KNO3
(v) CaCO3.
Listed below are the names of the compounds for each of the following formulae:
(i) Al2(SO4)3 – Aluminium sulphate
(ii) CaCl2 – Calcium chloride
(iii) K2SO4 – Potassium sulphate
(iv) KNO3 – Potassium nitrate
(v) CaCO3 – Calcium carbonate
What is meant by the term chemical formula?
Chemical formulas are used to describe the different types of atoms and their numbers in a compound or element. Each element’s atoms are symbolised by one or two letters. A collection of chemical symbols that depicts the elements that make up a compound and their quantities.
For example, the chemical formula of hydrochloric acid is HCl.
How many atoms are present in a
(i) H2S molecule and
(ii) PO43- ion?
The number of atoms present is as follows:
(i) H2S molecule has 2 atoms of hydrogen and 1 atom of sulphur hence 3 atoms in total.
(ii) PO43- ion has 1 atom of phosphorus and 4 atoms of oxygen hence 5 atoms in total.
Exercise-3.4
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