## Class 9 NCERT Atoms and Molecules Chap 3

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INTEXT QUESTIONS (Page No. 32–33)

Q.1.In a reaction, 5.3g of sodium carbonate reacted with 10 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

Ans: Mass of reactants = Mass of sodium carbonate + Mass of ethanoic acid.

= 5.3g + 6g = 11.3g

Mass of products = Mass of sodium ethanoate + Mass of carbon dioxide + Mass of water

= 8.2 g + 2.2 g + 0.9g = 11.3

The mass of reactants is equal to the mass of products. Hence, it proves law of conservation of mass.

Q. 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 : Oxygen

Ratio by mass 1 : 8

3g : ? Based on the ratio proposition.

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

Ans. Atoms are indivisible particles, which cannot be created nor be destroyed in a chemical reaction.

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

Ans. Atoms combine in the ratio of small whole numbers to from compounds.

INTEXT QUESTIONS (Page No. 35)

Q. 1. Define the atomic mass unit.

Ans. The mass equal to 1/12th of the mass of a atom is called one atomic mass unit.

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

Ans. An atom is an extremely small particle whose radius is of the order of 10–10 m. This size is so small that our eyes are not able to see it.

INTEXT QUESTIONS (Page No. 39)

Q. 1. Write down the formulae of (i) Sodium oxide (ii) Aluminium chloride (iii)Sodium sulphide (iv) Magnesium hydroxide

Q. 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) Aluminium sulphate (ii) Calcium chloride (iii) Potassium sulphate (iv) Potassium nitrate (v) Calcium carbonate.

Q. 3. What is meant by the term chemical formula?

Ans. It is a symbolic representation of its composition.

Q. 4. How many atoms are present in a : (i) H2S molecule and (ii) PO4 3– ion ?

Ans. (i) 3 atoms are present in H2S. (ii) 5 atoms are present in PO4 3– ion.

INTEXT QUESTIONS (Page No. 40)

Q. 1. Calculate the molecular masses of H2 , O2 , Cl2 , CO2 , CH4 , C2H6 , C2H4 , NH3 , CH3OH. [At mass of H = 1, O = 16, Cl = 35.5, C = 12, N = 14]

Ans. (i) Molecular mass of H2 = 2 × atomic mass of H = 2 × 1u = 2u

(ii) Molecular mass of O2 = 2 × atomic mass of O = 2 × 16u = 32u

(iii) Molecular mass of Cl2 = 2 × atomic mass of Cl = 2 × 35.5 u = 71u

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

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

(vi) Molecular mass of C2H6 = 2 × atomic mass of C + 6 × atomic mass of H = 2 × 12u + 6 × 1u = 30u

(vii) Molecular mass of C2H4 = 2 × atomic mass of C + 4 × atomic mass of H = 2 × 12u + 4 × 1u = 28u

(viii) Molecular mass of NH3 = 1 × atomic mass of N + 3 × atomic mass of H = 1 × 14u + 3 × 1u = 17u

(ix) Molecular mass of CH3OH = 1 × atomic mass of C + 4 × atomic mass of H + 3 × atomic of O

= 1 × 12u + 4 × 1u + 1 × 16u = 32u.

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

Ans. (i) Formula unit mass of ZnO = 1 × atomic mass of Zn + 1 × atomic mass of O = 1 × 65u + 1 × 16u

= 81u

(ii) Formula unit mass of Na2O= 2 × atomic mass of Na + 1 × atomic mass of O = 2 × 23u + 1 × 16u = 62u (iii) Formula unit mass of K2CO3= 2 atomic mass of k + 1 × atomic mass of C + 3 × atomic mass of O

= 2 × 39u + 1 × 12u + 3 × 16u = 78u + 12u + 48u = 138u.