IX(9th)Chemistry Ch 03:Atoms and Molecules Solved Questions

1. Avogadro’s number represents the number of atoms in

(a) 12g of C₁₂                                       (b) 320g of sulphur      

(c) 32g of oxygen                                (d) 12.7g of iodine

2. The number of moles of carbon dioxide which contain 8 g of oxygen is

(a) 0.5 mol                                           (b) 0.20 mol                              

(c) 0.40 mol                                         (d) 0.25 mol

(3) The total no of ions present in 111 g of cacl2 is

(a) One mole                                        (b) Two mole              

(c) Three mole                                     (d) Four moles

(4) Which of the following weighs the most ?

(a) one g-atom of nitrogen                   (b) One mole of water

(c) One mole of sodium                                   (d) One molecule of H2So4

(5) 5.0 litre of 0.4 M H2SO4 Contains-

(a) 2.0 Mole Of H2So4                                    (b) 0.4 mole H2So4

(c) 5.0 mole H2So4                             (d) 2.0 moles H2O

Ans: (1) a  (2) d (3) c (4) c (5) a

1. Find the ratio by mass of the combining elements in the compound – C2H5OH.

Ans: C = 2x12=24; H = 6x1=6 ; O= 1x16=16

C : H:O =24:6:16 = 12:3:8

2. Give the formula of the compound formed by the elements calcium and fluorine.

Ans:  Ca⁺² F^-1          CaF₂

3. What is the acid radical present in sodium peroxide?

Ans: The acid radical present in sodium peroxide (Na202) is peroxide radical (0₂^-2)

4. Carbon and silicon have the same valency. What is the formula of sodium silicate?

Valency of silicon and carbon is  4.

The formula of sodium silicate is Na+¹  SiO₃ ^-2  = Na₂ SiO₃

5. What is the ratio by number of atoms in mercurous chloride?

Ans: Formula of mercurous chloride is HgCI. Ratio of the atoms of Hg and CI in HgCI is 1: 1.

6. Name the element whose Latin name is Stibium.

Ans: Antimony (Latin:  stibium); Mercury (Latin: hydragyrum); Gold (Latin: aurum); Lead (Latin: plumbum).

7. What is the valency of a sulphide ion?

Ans: Valency of sulphide ion  So ₄ ^-2 is —2 eg.H₂SO₄

8. How many atoms of oxygen are present in 50g of CaCO3?

Molecular mass  of CaCO₃ = 40 + 12 + 3 X 16=100g
Atoms of oxygen are present  in 100 g of CaCO  = 3 x 6.022 x 10²³ atoms

Atoms of oxygen are present  in 50 g of CaCO  = {( 3 x 6.022 x 10²³) /100} x 50 atoms
                                                                         = 9.033 X 10²³

9. How many molecules are present in 1 ml of water?

Molecular mass of H₂O  =18gm  also, Mass of 1 mole of water=  1gm

18gm of water contain = 6.022 x 10²³ molecules

1gm of water contain = (6.022 x 10²³ )/18molecules = 3.34 x 10²²

10. What is the unit of measurement of atomic radius?

Ans: picometers (pm) or Angstroms (Å)