# NCERT Solution Class 9th Science Ch 3: Atoms and Molecules

NCERT Solution Class 9th Science Ch 3: Atoms and Molecules, NCERT Solutions for Class 9 Science (Chemistry) Chapter 3 – Atoms and Molecules solved by Expert Teachers as per NCERT (CBSE) Book guidelines. These solutions are part of NCERT Solutions for Class 9 Science. Here we have given Class 9 NCERT Science Text book Solutions for Chapter 3  Atoms and Molecules.

## NCERT Solution Class 9th Science Ch 3: Atoms and Molecules

#### 1: In a reaction 5.3 g of sodium carbonate reacted with 6 g of ethanoic acid. The products were 2.2 g of carbon dioxide, 0.9g water and 8.2 g of sodium ethanoate. Show that these observations are in agreement with the law of conservation of mass.

Sodium carbonate + ethanoic acid → sodium ethanoate + carbon dioxide + water.
Answer: In a reaction, sodium carbonate reacts with ethanoic acid to produce sodium ethanoate,
carbondioxide, and water.

Sodium + Ethanoic → Sodium + Carbon + Water
Carbonate acid ethanoate dioxide

Mass of sodium carbonate = 5.3g
Mass of ethanoic acid = 6g
Mass of sodium ethanoate = 8.2g
Mass of carbon dioxide = 2.2
Mass of water = 0.9g

Now, total mass before the reaction = (5.3 + 6)g
= 11. 3g
and total mass after the reaction = (8.2 + 2.2 + 0.9)g
= 11.3g
Therefore, Total mass before the reaction = Total mass after the reaction
Hence, the given 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 3g of hydrogen gas?

Answer: It is given that the ratio of hydrogen and oxygen by mass to form water is 1:8. Then, the mass of oxygen gas required to react completely with 1g of hydrogen gas is 8g. Therefore, the mass of oxygen gas required to react completely with 3g of hydrogen gas is 8 × 3g = 24 g.

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

Answer: The postulate of Dalton’s atomic theory which is a result of the law of conservation of mass is “Atoms are indivisible particles, which can neither be created nor destroyed in a chemical reaction”.

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

Answer: The postulate of Dalton’s atomic theory which can explain the law of definite proportion is “The relative number and kind of atoms in a given compound remains constant”.

### NCERT Solution Class 9th Science Ch 3: Atoms and Molecules

#### 1: Define atomic mass unit.

Answer: Mass unit equal to exactly one- twelfth the mass of one atom of carbon – 12 is called one atomic mass unit. It is written as ‘u’.

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

Answer: The size of an atom is so small that it is not possible to see it with naked eyes. Also, atom of an element does not exist independently.

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1: Write down the formula of
(i) sodium oxide
(ii) aluminium chloride
(iii) sodium suphide
(iv) magnesium hydroxide.

Answer: (i) Sodium oxide → Na2O
(ii) Aluminium chloride → AlCl3
(iii) Sodium suphide → Na2S
(iv) Magnesium hydroxide → Mg(OH)2

2: Write down the names of compounds represented by the following formula:
(i) Al2(SO4)3
(ii) CaCl2
(iii) K2SO4
(iv) KNO3
(v) CaCO3
(ii) CaCl2→ Calcium chloride
(iii) K2SO4→ Potassium sulphate
(iv) CaCO3→ Calcium carbonate.

#### 3: What is meant by the term chemical formula?

Answer: The chemical formula of a compound means the symbolic representation of the composition of a compound. From the chemical formula of a compound, we can know the number and kinds of atoms of different elements that constitute the compound. For example, from the chemical formula CO2 of carbon dioxide, we come to know that one carbon atom and two oxygen atoms are chemically bonded together to form one molecule of the compound, carbon dioxide.

#### 4: How many atoms are present in a

(i) H2S molecule and
(ii) PO43- ion?
Answer: (i) In an H2S molecule, three atoms are present; two of hydrogen and one of sulfur.
(ii) In a PO43- ion, five atoms are present; one of phosphorus and four of oxygen.

### Class 9th Science Ch 3 Long Type Solutions

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

Answer: Molecular mass of H2 = 2 × Atomic mass of H
= 2 × 1 = 2u
Molecular mass of O2 = 2 × Atomic mass of O
= 2 × 16 = 32u
Molecular mass of Cl2 = 2 × Atomic mass of Cl
= 2 × 35.5 = 71 u
Molecular mass of CO2 = Atomic mass of C + 2 × Atomic mass of O
= 12 + 2 × 16 = 44 u
Molecular mass of CH4 = Atomic mass of C + 4 × Atomic mass of H
= 12 + 4 × 1 = 16 u
Molecular mass of C2H6 = 2 × Atomic mass of C + 6 × Atomic mass of H
= 2 × 12 + 6 × 1 = 30u
Molecular mass of C2H4 = 2 × Atomic mass of C + 4 × Atomic mass of H
= 2 × 12 + 4 × 1 = 28u
Molecular mass of NH3 = Atomic mass of N + 3 × Atomic mass of H
= 14 + 3 × 1 =17 u
Molecular mass of CH3OH Atomic mass of C+4 ×Atomic mass of H+Atomic mass of O
= 12 + 4 × 1 + 16 = 32 u.

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

Answer: Formula unit mass of ZnO = Atomic mass of Zn + Atomic mass of O
= 65 + 16 = 81 u
Formula unit mass of Na2O = 2 × Atomic mass of Na + Atomic mass of O
= 2 × 23 + 16 = 62u
Formula unit mass of K2CO3
= 2 × Atomic mass of K + Atomic mass of C + 3 × Atomic mass of O
= 2 × 39 + 12 + 3 × 16 = 138u.

### Class 9th Science Ch 3 Problems

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

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

One mole of carbon atoms weighs 12g (Given)
i.e., a mass of 1 mole of carbon atoms = 12g
Then, the mass of 6.022× 1023 number of carbon atoms = 12g
Therefore, a mass of 1 atom of carbon = 12/ (6.022× 1023 )g
= 1.9926 × 10–23 g.

#### 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)?

The atomic mass of Na = 23u (Given)
Then, the gram atomic mass of Na = 23g
Now, 23g of Na contains = 6.022×1023 number of atoms
Thus, 100g of Na contains = (6.022×1023 × 100)/23 number of atoms
= 2.6182 × 1024 number of atoms.

Again, the atomic mass of Fe = 56u (Given)
Then, the gram atomic mass of Fe = 56g
Now, 56 g of Fe contains = 6.022×1023 number of atoms
Thus, 100 g of Fe = (6.022×1023 × 100)/56 number of atoms
= 1.0753 × 1024 number of atoms.
Therefore, 100 grams of sodium contains more atoms than 100 grams of iron.

### Exercises

#### 1: A 0.24 g sample of a compound of oxygen and boron was found by analysis to contain 0.096 g ofboron and 0.144 g of oxygen. Calculate the percentage composition of the compound by weight.

Mass of boron = 0.096g
Mass of oxygen = 0.144g
Mass of sample = 0.24g
Thus, the percentage of boron by weight in the compound = 0.096×100/0.24 =40%
Thus, the percentage of oxygen by weight in the compound =0.144×100/0.24 =60%

#### 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 combinations will govern your answer?

Answer: Carbon + Oxygen → Carbon dioxide
3g of carbon reacts with 8 g of oxygen to produce 11g of carbon dioxide. If 3g of carbon is burnt in 50g of oxygen, then 3g of carbon will react with 8 g of oxygen. The remaining 42 g of oxygen will be left un-reactive. In this case, also, only 11g of carbon dioxide will be formed. The above answer is governed by the law of constant proportions.

#### 3: What are polyatomic ions? Give examples?

Answer: A polyatomic ion is a group of atoms carrying a charge (positive or negative).
For example, ammonium ion (NH +4), hydroxide ion (OH−), carbonate ion (CO32−),
sulfate ion ( SO42−).

### Class 9th Science Ch 3: Atoms and Molecules Very Short Type Question Answer

See Also: NCERT Solutions for Class 9th: Ch 3 Atoms and Molecules

4: Write the chemical formula of the following:
(a) Magnesium chloride
(b) Calcium oxide
(c) Copper nitrate
(d) Aluminum chloride
(e) Calcium carbonate
(a) Magnesium chloride →MgCl2
(b) Calcium oxide →CaO
(c) Copper nitrate →Cu(NO3)2
(d) Aluminum chloride →AlCl3
(e) Calcium carbonate →CaCO3

5: Give the names of the elements present in the following compounds:
(a) Quick lime
(b) Hydrogen bromide
(c) Baking powder
(d) Potassium sulfate.

6: Calculate the molar mass of the following substances:
(a) Ethyne, C2H2
Answer: Molar mass of ethyne, C2H2 = 2 × 12 + 2 × 1 = 28g.

(b) Sulphur molecule, S8
Answer: Molar mass of sulphur molecule, S8 = 8 × 32 = 256g.

(c) Phosphorus molecule, P4 (atomic mass of phosphorus = 31)
Answer: Molar mass of phosphorus molecule, P4 = 4 × 31 = 124g.

(d) Hydrochloric acid, HCl
Answer: Molar mass of hydrochloric acid, HCl = 1 + 35.5 = 36.5g.

(e) Nitric acid, HNO3
Answer: Molar mass of nitric acid, HNO3 = 1 + 14 + 3 × 16 = 63g.

7: What is the mass of
(a) 1 mole of nitrogen atoms?
Answer: The mass of 1 mole of nitrogen atoms is 14g.

(b) 4 mole of aluminum atoms (Atomic mass of aluminum = 27)?
Answer: The mass of 4 moles of aluminum atoms is (4 × 27)g = 108g.

(c) 10 moles of sodium sulfite (Na2SO3)?
Answer: The mass of 10 moles of sodium sulphite (Na2SO3) is 10 × [2 × 23 + 32 + 3 × 16]g
= 10 × 126g = 1260g

8: Convert into mole.
(a) 12g of oxygen gas
Answer: 32 g of oxygen gas = 1 mole
Then, 12g of oxygen gas = 12/32 mole = 0.375 mole.

(b) 12g of water
Answer: 18g of water = 1 mole
Then, 20 g of water = 20/18 mole = 1.11 moles (approx.)

(c) 22g of carbon dioxide
Answer: 44g of carbon dioxide = 1 mole
Then, 22g of carbon dioxide = 22/44 mole = 0.5 mole.

9: What is the mass of:
(a) 0.2 mole of oxygen atoms?
Answer: Mass of one mole of oxygen atoms = 16g
Then, mass of 0.2 mole of oxygen atoms = 0.2 × 16g = 3.2g.

(b) 0.5 mole of water molecules?
Answer: Mass of one mole of water molecule = 18g
Then, mass of 0.5 mole of water molecules = 0.5 × 18g = 9g.

10: Calculate the number of molecules of sulfur (S8) present in 16g of solid sulfur.
1 mole of solid sulphur (S8) = 8 × 32g = 256g
i.e., 256g of solid sulphur contains = 6.022 × 1023 molecules
Then, 16g of solid sulfur contains = (6.022× 1023 / 256 )×16 molecules
= 3.76 × 1022 molecules (approx).

11: Calculate the number of aluminum ions present in 0.051g of aluminum oxide.
(Hint: The mass of an ion is the same as that of an atom of the same element. The atomic mass of Al = 27u)