# KSEEB Solutions for Class 8 Science Chapter 4 Atoms and Molecules

## Karnataka Board Class 8 Science Chapter 4 Atoms and Molecules

### KSEEB Class 8 Science Chapter 4 Textual Questions & Answers

I. Four alternatives are given to each of the following incomplete statements. Choose the right answer.

Question 1.
Some elements have atomic mass in fractions. It is due to the presence of
(a) fractions of protons
(b) fractions of neutrons
(c) isotopes
(d) valence electrons.
(c) isotopes

Question 2.
Plants in a garden make use of 22.4 liters of carbon-dioxide at STP. The numbers of molecules of carbon-dioxide utilized by the plant is
(a) 6.023 x 1023
(b) 22.4 molecules
(c) 6.023 x 1024
(d) 11.2 moles of molecules
(a) 6.023 x 1023

Question 3.
The atomic mass of deuterium isotope of hydrogen is 2. The atomic mass of oxygen is 16. Then the molecular mass of heavy water formed is
(a) 16
(b) 18
(c) 20
(d) 24
(c) 20

Question 4.
Some elements can have different valencies. It can happen due to the change in
(a) of electrons that participate in a reaction.
(b) Neutron number
(c) Proton number
(d) atomic mass number
(a) of electrons that participate in a reaction.

II. Give scientific reason for the following:

Question 1.
The valency of noble gases is zero.
Noble gases are 18th group elements which have either completed shell of electrons or 8 electrons in the outermost shell. They do not have tendency to react with other elements to form compounds.

Question 2.
In the atmosphere, Oxygen is available as O2 but not as O.
Oxygen is written as O2 when there are two oxygen atoms in the molecule pure oxygen doesn’t generally exist as individual atoms, two oxygen atoms bond together to form an oxygen molecule. This is because oxygen is highly reactive disconnected atoms have a very strong tendency to form bonds. If there’s nothing else available they will bond to each other.

Question 1.
Define :
1) Atomic mass
2) Relative atomic mass
3) Gram molecular mass
4) Mole
5) Valency of an element

1. Atomic mass: The mass of an atom of a chemical element is expressed in atomic mass units. It is approximately equivalent to the total number of protons and neutrons in the atom (the mass number) or to the average number allowing for the relative abundance of different isotopes.

2. Relative atomic mass: The ratio of the average mass of one atom of an element to one-twelfth of the mass of an atom of carbon-12 is the relative atomic mass.

3. Gram molecular mass: A mass in grams numerically equal to the molecular weight of a substance or the sum of all the atomic masses in its molecular formula.

4. Mole: Mole is the amount of pure substance containing the same number of chemical units as there are atoms in exactly 12 grams of carbon-12 (ie 6.023 x 1023)

5. Valency of elements: Valley of elements is a measure of its combining power with other atoms when it forms chemical compounds or molecules.

Question 2.
Calculate the percentage composition of Carbon and Oxygen in  CO2 , (given atomic masses : Carbon = 12 and Oxygen = 16).
Mass of carbon dioxide [CO2]
= 1 (C) + 2(0)
= 1(12) + 2(16)
= 12 + 32 = 44
% mass of carbon = $$\frac { 12 }{ 44 }$$ × 100 = 21.3%
% mass of oxygen = $$\frac { 32 }{ 44 }$$ × 100 = 72.7%

Question 3.
A student has a piece of copper sulfate (CuSO4) crystal. Explain the method of calculating the number of CuSO4  molecules in that crystal. (Given atomic masses : Copper = 40, Sulfur = 32 and Oxygen = 16)
Molecular formula of copper sulfate = CuSO4
Gram molecular mass of CuSO4 = 40+32 + (16 × 4) = 72 + 64 = 136 grams
Gram molecular mass of CuSO4 = 1 mole = 6.023 × 1023 molecules
If the students has 136 grams of CuSOthen the number of molecules in that crystal will be 6.023 × 1023

Question 4.
CaC03 $$\xrightarrow { heat }$$ CaO + CO2
From the above equation, calculate the amount of carbon dioxide in grams liberated by heating 25 g of calcium carbonate.
CaCO3$$\xrightarrow { heat }$$ CaO + CO2 ↑
(40 × 1)+(1 × 12) + (16 × 3) → (40 + 1) + (16 × 1) + (12 × 1) + (16 × 2) 40 + 12+48 → (40 + 16) + (12 + 32)
100 gms of CaCO3 → 56 gm of CaO + 44 gms of CO2
When 100 gms of CaCO3 is heated 44 gms of CO2 is liberated.
Therefore when 25 gms of CaCO3 is heated the amount of CO2 liberated = 11 gms

I. Fill in the blanks with suitable words:

Question 1.
Modem standard for measuring atomic mass is $$\frac { 1 }{ 12 }$$ mass of ………… isotope
6C12

Question 2:
The average atomic mass of chlorine is …………
35

Question 3.
The molecular mass of water is …………
18

Question 4.
Gram molecular volume of a gas at STP ………….. liter.
22.4

Question 1.
A student writes “preparation of CO2 in the laboratory in his book. From the point of view of chemistry, what is the mistake made in writing?
From the point of view of the mole concept, he/she should have written how many moles of CO2 he/she is going to prepare.

III. Match the following: 1. (f)
2. (a)
3. (d)
4. (e)

Question 1.
Define atomic mass unit (a.m.u)
one-twelth of mass of the atom 6C12 isotope is taken as a standard to compare the mass of other atoms and molecules this unit is called atomic mass unit.

Question 2.
How much is one a.m.u?
One a.m.u = 1.66 x 10-24 g
= 1.66 x 10-27 kg
$$\frac { 1 }{ 12 }$$ of the mass of one atom of carbon
6C12 is one a. m. u. (Atomic Mass Unit)
i.e. 1.66 × 10-24 g

Question 3.
What is meant by relative atomic mass of an element?
The relative atomic mass of an element is the ratio of the mass an atom of the element to
one-twelfth of the mass of an atom of carbon 6 c Question 4.
What is meant by the relative molecular mass of a substance?
The relative molecular mass of a substance is the ratio of the mass of a molecule of
the substance to $$\frac { 1 }{ 12 }$$ the mass of an atom of Carbon 6C12 isotope.

Question 5.
Write the relative atomic masses of some elements.

 Element Symbol Relative mass Aluminum Al 26.98 Calcium Ca 40.08 Carbon C 12.01 Chlorine Cl 35.45 Hydrogen H 1.008 Magnesium Mg 24.31 Nitrogen N 14.01 Oxygen 0 15.99 Silver Ag 107.57

Question 6.
Calculate the relative molecular mass of carbon dioxide.
Molecular formula of carbon dioxide =CO2
Atomic mass of Carbon = 12
Atomic mass of Oxygen = 16
1 × atomic mass of carbon + 2 × atomic mass of oxygen.
= (1 × 12) +(2 × 16) = 44
Hence relative molecular mass of carbon dioxide = 44.

Question 7.
What is meant by gram atomic mass? Give example.
Ans. Gram atomic mass of an element is its relative atomic mass expressed in grams
Ex :
(1)The gram atomic mass of a hydrogen atom is 1. 008 g.
(2) The gram atomic mass of an oxygen atom is 16 g.

Question 8.
What is meant by gram molecular mass ? Give an example.
Gram molecular mass of a substance is its relative molecular mass expressed in gram’s
Ex :
1. The gram molecular mass of hydrogen is 2 g
2. The gram molecular mass of chlorine is 71 g.

Question 9.
Write the molecular mass of some substances. Question 10.
What is meant by a mole? Explain.
A ‘mole’ represents a collection of 6.023 × 1023 atoms or molecules. A “mole” represents the number of atoms or molecules in 22.4 liters of any gas at standard temperature and pressure.
The volume 22.4 liters is also called gram molecular volume. A ‘mole’ is equal to 6.023 × 1023 atoms or molecules. This is also called Avogadro’s number and is represented by ‘N’.

Question 11.
Mention the signification of mole.
Signification mole
(1) mole concept gives us a method of calculating the number of atoms present in a given mass of a substance.
(2) It helps in the calculation of the ratio of reactions consumed and products formed quantitatively.

Question 12.
To prepare one gram of hydrogen molecules using hydro caloric acid how many grams of zinc should be taken?
Zn + 2HCl → ZnCl2+H2
Gram atomic mass of zinc is = 65 g and Gram molecular mass of Hydrogen = 2 g From the above equation we can understand that 65 g of zinc gives 2 g of hydrogen molecules. To prepare 1 g of hydrogen molecules we need 32.5 g of zinc.

Question 13.
What are inert gases or noble gases? Give example.
Some elements do not have the tendency to react with other elements to form compounds. They are called inert gases or noble gases.
Ex: Helium, Neon, Argon.
[Zero valency elements] or [Inner gases]

Question 14.
What is meant by valency? Give an example.
Valency can be considered as the Combining capacity of an element.
Ex:

 Elements Symbol Oxygen 2 Sodium 1 Silicon 2 Nitrogen 5 Sulfur 2

Question 15.
What is meant by variable valency?
The same element may exhibit different valencies in different compounds and this property is called as variable valency.
Ex: Iron has 2 valencies.
Ferric has 3 valencies It is because the number of electrons from the iron atoms which participate in the chemical reaction will be either 2 or 3 depending upon conditions.

Question 16.
Write the common features of the Noble gases?