Access Answers of Science NCERT Class 9 Chapter 3 – Atoms and Molecules (All in text and Exercise Questions solved)

Class 9 Science Chapter 3 Exercise-3.1 Questions with Answer

Exercise-3.1 Page: 32

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 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:

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, LHS = RHS    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.

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:

We know hydrogen and water mix in the 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.

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

Solution:

The postulate of Dalton’s Atomic theory which is a result of the law of conservation of mass is,

“Atoms can neither be created nor destroyed”.

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

Solution:

The postulate of Dalton’s atomic theory that can explain the law of definite proportions is – the

relative number and kinds of atoms are equal in given compounds.

Exercise-3.2 Page: 35

1. Define the atomic mass unit?

Solution:

An atomic mass unit is a unit of mass used to express weights of atoms and molecules where one

atomic mass is equal to 1/12th the mass of one carbon-12 atom.

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

Solution:

Firstly, atoms are miniscule in nature, measured in nanometers. Secondly, except for atoms of noble

gasses, they do not exist independently. Hence, an atom cannot be visible to the naked eyes.

Exercise-3.3-3.4 Page: 39

1. Write down the formulae of

(i) sodium oxide

(ii) aluminium chloride

(iii) sodium sulphide

(iv) magnesium hydroxide

Solution:

The following are the formulae:

(i) sodium oxide – Na2O

(ii) aluminium chloride – AlCl3

(iii) sodium sulphide – Na2S

(iv) magnesium hydroxide – Mg (OH)2

2. Write down the names of compounds represented by the following formulae:

(i) Al2(SO4)3

(ii) CaCl2

(iii) K2SO4

(iv) KNO3

(v) CaCO3.

Solution:

Listed below are the names of the compounds for each of the following formulae

(i) Al2(SO4)3 – Aluminium sulphate

(ii) CaCl– Calcium chloride

(iii) K2SO4 – Potassium sulphate

(iv) KNO– Potassium nitrate

(v) CaCO3 – Calcium carbonate

3. What is meant by the term chemical formula?

Solution:

Chemical formula is the symbolic representation of a chemical compound. For example: The chemical formula of hydrochloric acid is HCl.

4. How many atoms are present in a

(i) H2S molecule and

(ii) PO43- ion?

Solution:

The number of atoms present are as follows:

(i) H2S molecule has 2 atoms of hydrogen and 1 atom of sulphur hence 3 atoms in totality.

(ii) PO43- ion has 1 atom of phosphorus and 4 atoms of oxygen hence 5 atoms in totality.

Exercise-3.5.1-3.5.2 Page: 40

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

Solution:

The following are the molecular masses:

The molecular mass of H2 – 2 x atoms atomic mass of H = 2 x 1u = 2u

The molecular mass of O2 – 2 x atoms atomic mass of O = 2 x 16u = 32u

The molecular mass of Cl2 – 2 x atoms atomic mass of Cl = 2 x 35.5u = 71u

The molecular mass of CO2 – atomic mass of C + 2 x atomic mass of O = 12 + ( 2×16)u = 44u

The molecular mass of CH4 – atomic mass of C + 4 x atomic mass of H = 12 + ( 4 x 1)u = 16u

The molecular mass of C2H6– 2 x atomic mass of C + 6 x atomic mass of H = (2 x 12) +

(6 x 1)u=24+6=30u

The molecular mass of C2H4– 2 x atomic mass of C + 4 x atomic mass of H = (2x 12) +

(4 x 1)u=24+4=28u

The molecular mass of NH3– atomic mass of N + 3 x atomic mass of H = (14 +3 x 1)u= 17u

The molecular mass of CH3OH – atomic mass of C + 3x atomic mass of H + atomic mass of O + atomic mass of H = (12 + 3×1+16+1)u=(12+3+17)u = 32u

2. Calculate the formula unit masses of ZnO, Na2O, K2CO3, given atomic masses of Zn = 65u,

Na = 23 u,  K=39u, C = 12u, and O=16u.

Solution:

Given:

Atomic mass of Zn = 65u

Atomic mass of Na = 23u

Atomic mass of K = 39u

Atomic mass of C = 12u

Atomic mass of O = 16u

The formula unit mass of ZnO= Atomic mass of Zn + Atomic mass of O = 65u+16u = 81u

The formula unit mass of Na2O = 2 x Atomic mass of Na + Atomic mass of O = (2 x 23)u +16u=46u+16u=62u

Exercise-3.5.3 Page: 42

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

Solution:

Given: 1 mole of carbon weighs 12g

1 mole of carbon atoms = 6.022 X 1023

Molecular mass of carbon atoms = 12g = an atom of carbon mass

Hence, mass of 1 carbon atom = 12 / 6.022 X 1023  = 1.99 X 10-23g

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

Solution:

Given: Atomic mass of Na=23u, Atomic mass of Fe= 56u

To calculate the number of atoms in 100g of sodium:

23g of Na contains = 6.022 X 1023 atoms

1g of Na contains = 6.022 X 1023 atoms / 23

100g of Na contains = 6.022 X 1023 atoms X 100 / 23

= 2.6182 X 1024 atoms

To calculate the number of atoms in 100g of sodium:

56g of Fe contains = 6.022 X 1023 atoms

1g of Fe contains = 6.022 X 1023 atoms / 56

100g of Fe contains =  6.022 X 1023 atoms X 100 / 56

= 1.075 X 1024 atoms

Hence, through comparison, it is evident that 100g of Na has more atoms.

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