Atoms and Molecules NCERT in-text questions

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Hello readers ! Here, class 9, Chapter – 03 Atoms and Molecules NCERT in-text questions are given.

Atoms and Molecules NCERT in-text questions

NCERT in-text questions page 32/33

Question 1:  

In a reaction, 5.3 g 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

Answer: In above question, Mass of reactants = Sodium carbonate + Acetic acid = 5.3 + 6 = 11.3 gm

Mass of products = Sodium acetate + Carbon dioxide + Water  = 2.2 + 0.9 + 8.2 = 11.3 gm

From above it is clear that, the mass of reactant = mass of the product.

Hence, given observations are in agreement with the law of conservation of mass.

Concept ☞ Law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction. i.e. In a chemical reaction mass of reactant should be equal to mass of product.

Question 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?

Ans:  Hydrogen and oxygen combine in the ratio of 1:8 by mass to form water there ratio is always constant.

1g of water contains of about 0.11 grams of hydrogen and 0.88 grams of oxygen to maintain hydrogen and oxygen ratio 1:8.

Since in water, every 1g of hydrogen needed 8g of oxygen.

So, 3g of hydrogen require 3 x 8 = 24g of oxygen.

Concept ☞ law of constant proportions states that “In a chemical substance the elements are always present in definite proportions by mass”.

Question 3:

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

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

• Concept ☞ According to the postulates of Dalton’s atomic theory,
  • (i) All matter is made of very tiny particles called atoms, which participate in chemical reactions.
  • (ii) Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.
  • (iii) Atoms of a given element are identical in mass and chemical properties.
  • (iv) Atoms of different elements have different masses and chemical properties.
  • (v) Atoms combine in the ratio of small whole numbers to form compounds.
  • (vi) The relative number and kinds of atoms are constant in a given compound.

Question 4:

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

Ans: The postulate of Dalton’s atomic theory which can explain the law of definite proportions is ” The relative number and kinds of atoms are constant in a given compound”.

NCERT in-text questions page 32/33

Question 1:

Define the atomic mass unit.

Ans: One atomic mass unit is a mass unit equal to exactly one-twelfth (1/12th) the mass of one atom of carbon-12.

Question 2:

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

Ans: Size of atom is very small about 10^-10m however a normal human eye can see object of size upto 10^-4m only.

NCERT in-text questions page 39

Question 1:

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

Ans: (i) Sodium oxide – Na₂O

(ii) Aluminium chloride – AlCl₃

(iii) Sodium sulphide – Na₂S

 (iv) Magnesium hydroxide – Mg(OH)₂

Ans: (i) Sodium oxide – Na2O

(ii) Aluminium chloride – AlCl3

(iii) Sodium sulphide – Na2S

 (iv) Magnesium hydroxide – Mg(OH)2

Question 2:

Write down the names of compounds represented by the following formulae:
(i) Al2 (SO4)3 (ii) CaCl2 (iii) K2SO4 (iv) KNO3 (v) CaCO3.

Ans: (i) Al₂ (SO₄)₃ – Aluminium sulfate

(ii) CaCl₂ – Calcium chloride

(iii) K₂SO₄ – Potassium sulfate

(iv) KNO₃ – Potassium Nitrate

(v) CaCO₃ – Calcium carbonate

Question 3:

What is meant by the term chemical formula?

Ans: Chemical formula is a representation in which composition elements of a compound / molecule is represented in the form of symbols also the ratio in which the atoms of these elements combine to form such compound / molecules.

Question 4: 

How many atoms are present in a (i) H₂S molecule and (ii) PO₄^3– ion?

Ans: (i) H₂S molecule – total 3 atoms, two atoms of hydrogen and one atom of Sulphur.

(ii) PO₄^3– ion – total 5 atoms, one atom of phosphorus and four atoms of oxygen

NCERT in-text questions page 40

Question 1:

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

Ans: Molecular masses of H₂ = 2 ×Atomic mass of H= 2 ×1 u = 2 u

Molecular masses of O₂ = 2 ×Atomic mass of O= 2 ×16 u = 32 u

Molecular masses of Cl₂ = 2 ×Atomic mass of Cl= 2 ×35.45 u = 70.90 u

Molecular masses of CO2 = (1 ×Atomic mass of C) + (2 × Atomic mass of O) = (1 ×12 u) + (2 ×16 u) = 44 u

Molecular masses of CH₄ =  (1 ×Atomic mass of C) + (4 × Atomic mass of H) = (1 ×12 u) + (4 ×1 u) = 16 u

Molecular masses of C₂H₆ =  2 ×Atomic mass of C + 6 × Atomic mass of H= 2 ×12 u + 6 ×1 u = 30 u

Molecular masses of C₂H₄ =  2 ×Atomic mass of C + 4 × Atomic mass of H= 2 ×12 u + 4 ×1 u = 28 u

Molecular masses of NH₃ =  1 × Atomic mass of N + 3 × Atomic mass of H= 1 ×14 u + 3 ×1 u = 17 u

Molecular masses of CH₃OH =  (1 × Atomic mass of C) + (3 × Atomic mass of H) + (1 × Atomic mass of O) + (1 × Atomic mass of H) = (1 ×12 u) + (3 ×1 u) + (1 ×16 u) + (1 ×1 u) = 32 u

Question 2:  

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

Ans: Formula unit masses of ZnO = (1× atomic mass of Zn) + (1× atomic mass of O) = ( 1 × 65 u ) + ( 1 × 16 u ) = 81 u

Formula unit masses of Na₂O = (2× atomic mass of Na) + (1× atomic mass of O) = ( 2 × 23 u ) + ( 1 × 16 u ) = 62 u

Formula unit masses of K₂CO₃ = (2× atomic mass of K ) + (1× atomic mass of C ) + (3× atomic mass of O ) = ( 2 × 39 u ) + ( 1 × 12 u ) + ( 3 × 16 u ) = 138 u

39 u ) + ( 1 × 12 u ) + ( 3 × 16 u ) = 138 u

NCERT in-text questions page 42

Question 1:

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

Ans: Concept ☞ Number of atoms in 1 mole of carbon is equal to Avogadro Number = 6.022× 10²³ atoms  

Mass of 1 mole of carbon atoms = 12g

Mass of 6.022× 10²³ number of carbon atoms = 12g

So, mass of 1 atom of carbon = 12/6.022×10²³ g = 1.99 ×10^–²³ g

Question. 2:

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

Ans:  Number of atoms in 100 grams of sodium = ?

Number of moles (n) in 100 gram mass of sodium = mass of sodium (m) / molecular mass of sodium (M) = 100 / 23

Number of atoms in 100 grams of sodium = Number of moles (n) in 100 gram mass of sodium × 6.022 × 10²³ atoms = (100/23) × 6.022 × 10²³ atoms = 2.618 × 10²⁴ atoms

Number of atoms in 100 grams of Iron = ?

Number of moles (n) in 100 gram mass of Iron = mass of Iron (m) / molecular mass of Iron (M) = 100 / 56

Number of atoms in 100 grams of Iron = Number of moles (n) in 100 gram mass of Iron × 6.022 × 10²³ atoms = (100/56) × 6.022 × 10²³ atoms = 1.0735 × 10²⁴ atoms

Comparing above it is clear that 100 grams of Sodium has more number of atoms then 100 grams of Iron.

• Concept ☞ Number of moles (n) in m gram mass of atoms = mass of atoms (m) / molecular mass (M)
• Number of atoms in 1 mole of atoms = Avogadro Number = 6.022× 10²³ atoms
• Number of atoms in n mole of atoms = n × Avogadro Number = n × 6.022 × 10²³ atoms

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