KSEEB Solutions for Class 8 Science Chapter 4 Atoms and Molecules

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Karnataka State Syllabus Class 8 Science Chapter 4 Atoms and Molecules

KSEEB Class 8 Science Atoms and Molecules Textbook Exercise Questions and Answers

I. Four alternatives are given to each of the following incomplete statements/questions. 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
Answer:
(c) isotopes

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

Question 3.
The atomic mass of Deuterium an 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
Answer:
(c) 20

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

Answer:
1. \(1 \frac{1^{\text {th }}}{12}\) mass of c12 isotope
2. 35.45,
3. 18
4. 22.4

II. Give scientific reason

(a) The valency of noble gases is zero.
Valency is the combining capacity of an element. Inert gases have a completely filled valence shell and do not combine with other elements to form compounds. Hence the valency of inert gases is zero.

(b) In the atmosphere, oxygen is available as O₂ but not as O.
Valency of oxygen is 2 because it can accept or share 2 electrons to form a bond. Hence it is available as O₂ but not as O.

III. Answer the following:

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

Atomic mass : Atomic mass can be defined as the total mass of its nucleus
Relative atomic mass : 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 6^C12 isotope.
Gram molecular mass : Gram atomic mass of an element is its relative atomic mass expressed in grams
Mole : A ‘mole’ is equal to 6.023 × 10²³ atoms or molecular mass expressed in grams
Valency of an element :- It is the combining capacity of that element.

Question 2.
Calculate the percentage composition of carbon and oxygen CO₂ (given atomic masses carbon-12 and oxygen-16)
Answer:
Divide their individual weights by the total weight, expressed as a percentage.
Total weight of CO₂ = 12 x 1 + 16 x 2 = 12 + 32 = 44
% of C in CO₂ = 12/44 x 100 = 0.272 x 100 = 27.27%
% of oxygen in CO₂ = 32/44 x 100 =
0. 7272 x 100 = 72.72%

Question 3.
Using a piece of calcium sulfate a student writes the ‘mole’ Explain briefly the method of calculation of finding the number of calcium sulfate molecules used to write the word mole. (Data atomic masses : Calcium = 40, Sulphur = 32, Oxygen = 16)
Answer:
The molecular formula of calcium sulfate is CaSO₄
The gram molecular mass of CaSO₄ 40 + 32 + (16 x 4)
= 40 + 32 + 64
= 136 grams

Gram molecular mass of CaSo₄ = 1 mole ⇒ 6.023 x 10²³ molecules
If the student has used 136 grams of CaSO₄ then the number of molecules will be 6.023 x 10²³

Question 4.
CaCO₃ \(\underrightarrow { Heat }\) CaO + CO₂ ↑
From the above equation, calculate the amount of carbon dioxide in grams liberated by heating 25 gms of calcium carbonate.
CaCO₃ \(\underrightarrow { Heat }\) CaO + CO₂ ↑
CaCO₃ \(\underrightarrow { Heat }\) CaO₂ ↑ {(40 x 1)+ (12 x 1)+ (16 x 3)} + {(40 x 1) + (16 + 1)} + {(12 x 1) + (16 x 2)}
40 + 12 + 48 → (40 + 16) + 12 + 32
100 gms of CaCO₃ → 56 gms of CaO + 44 gms of CO₂
When 100 gms of CaCO₃ is heated 44 gms of CO₂ is liberated
Therefore when 25 gms of Ca CO₃ is heated the amount of CO₂ liberated
\(\frac{44 \times 25}{100}=\frac{44}{4}\) = 11 gms.

IV. Match the following:

Answers:
1. f
2. a
3. d
4. e.

KSEEB Class 8 Science Atoms and Molecules Additional Questions and Answers

Question 1.
What is meant by the relative atomic mass of an element?
Answer:
The relative atomic mass of an element is the ratio of the mass of an atom of the element to one-twelfth of the mass of an atom of Carbon ₆C¹².
Relative atomic mass = \(\frac{\text { Mass of } 1 \text { atam of the element }}{\frac{1}{12} \times \text { mass of } 1 \text { atom of }_{6} \mathrm{C} \text { isotope }}\)

Question 2.
What is meant by the relative molecular mass of a substance?
Answer:
The relative molecular mass of a substance is the ratio of the mass of a molecular of the substance to \(\frac{1}{12}\) the mass of an atom of carbon ₆C¹².
Relative molecular mass = \(\frac{\text { Mass of } 1 \text { mokaule of the substance }}{\frac{1}{12} \times \text { mass of } 1 \text { atom of } \delta C^{12}}\)

Question 3.
What is meant by Gram Atomic mass?
Answer:
The gram atomic mass of an element is its relative atomic mass expressed in grams.

Question 4.
What is the mole concept? What is its utility?
Answer:
A mole represents a collection of 6.022 x 0²³ entitles (atoms, molecules, ions, electrons, etc.) which is the gram atomic or molecular mass of an element compound, (or) Mole is defined as the quantity of the substance that contains Avagadro number of a particular species.

The signification of mole:

It gives the method of calculating the number of atoms present in a given mass of a substance.
It helps in the calculations of the ratio of reactants consumed and products formed quantitatively.

Question 5.
What is meant by valency?
Answer:
Valency can be considered as the combining capacity of an element.

Question 6.
What is the modern concept of valency?
Answer:
The modern concept of valency can be defined as the number of electrons of an element that take part in chemical reactions.

Question 7.
What are inert gases or noble gases?
Answer:
Some elements do not have a natural tendency to react with other elements to form compounds. These elements valency is zero since they have either completed shell of electrons or 8 electrons in the outermost shell (octet structure) Example Helium, Neon, Argon, etc.

Question 8.
What is meant by variable valency?
Answer:
The same element may exhibit different valencies in the different compounds and participation of electrons of the same element may vary depending upon conditions]. This property is called as variable valency.

Fill in the blanks:

The unit used to measure the atomic mass is ________________.
The amu is equal to ________________ gms.
A mole is, equal to ________________ atoms or molecules
Gram atomic mass of a substance contains ________________ atoms.
The standard for determining the atomic mass of element is the isotope ________________.
One mole of NH₃ weighs ________________ gms.
One mole of oxygen at STP occupies a volume of ________________ litres.
One mole of hydrogen contains ________________ mniecules.
\(\frac{1}{12}\) the mass of one C¹² atom is known as ________________.
Mass of one carbon atom is ________________.

Answers:

A.m.u,
1.66 x 10^-24 gms.
6.023 x 10²³
6.023 x 10²³
C¹²
7
22.4
6.023 x 10²³
A.m.u,
1.99 x 10^-23

Match the following:

A – B
1. Mole – a. Kelvin
2. Avagadro No. – b. 8 electrons
3. Octet structure – c. Helium
4. Noble gas – d. Avogadro
Answers:
1. d
2. a
3. b
4. c

Choose the most appropriate option:

Question 1.
Gram molecular mass of CO₂ is
(a) 32 gms
(b) 44 gms
(c) 24 gms
(d) 48 gms
Answer:
(b) 44 gms

Question 2.
22.4 litres of oxygen at STP weighs,
(a) 16 gms
(b) 32 gms
(c) 64 gms
(c) 128 gms
Answer:
(b) 32 gms

Question 3.
Which one contains the largest number of molecules?
(a) 0.2 moles of H₂
(b) 8.0 gms of H₂
(c) 16.0 gms of HO₂
(d) 10.7 gms of H₂O
Answer:
(b) 8.0 gms of H₂

Question 4.
Relative mass of one mole H₂SO₄ is
(a) 44 gm
(b) 98.gm
(c) 32 gm
(d) 100 gm
Answer:
(b) 98.gm

Question 5.
The electronic configuration of an atom is 1.5 The number of 2-8-2 valence electrons in the atom is
(a) 10
(b) 12
(c) 8
(d) 2
Answer:
(d) 2