CBSE - Chemistry - Atoms and Molecules

1. Which of the following correctly represents 360 g of water?
(i) 2 moles of H₂0
(ii) 20 moles of water
(iii) 6.022 � 10²³ molecules of water
(iv) 1.2044�10²⁵ molecules of water
(a) (i)
(b) (i) and (iv)
(c) (ii) and (iii)
(d) (ii) and (iv)
Ans. (d) (ii) and (iv)

2. Which of the following statements is not true about an atom?
(a) Atoms are not able to exist independently
(b) Atoms are the basic units from which molecules and ions are formed
(c) Atoms are always neutral in nature
(d) Atoms aggregate in large numbers to form the matter that we can see, feel or touch
Ans. (a) Atoms are not able to exist independently.
Explanation: Atoms of inert gases exist independently, i.e. as single atom.


3. The chemical symbol for nitrogen gas is
(a) Ni
(b) N₂
(c) N⁺
(d) N
Ans. (b) N₂

4. The chemical symbol for sodium is
(a) So
(b) Sd
(c) NA
(d) Na
Ans. (d) Na

5. Which of the following would weigh the highest?
(a) 0.2 mole of sucrose (C₁₂H₂₂O₁₁)
(b) 2 moles of CO₂
(c) 2 moles of CaCO₃
(d) 10 moles of H₂O
Ans. (c) 2 moles of CaCO₃

Explanation: Weight of 1 mole of each compound can be calculated as follows:

(a) Sucrose =  12 � 12+  22 � 1 +11 � 16 = 144  + 22  +  176 =342 g
Or 0.2 M Sucrose =  342 � 0.2  = 68.4 g
(b) 1 M carbon dioxide =    1 � 12 + 2 � 16  = 44 g
Or 2 M CO₂ = � = 44 2 88 g
(c) 1 M CaCO₃ =    1 � 10 +  1  � 12 + 3 � 16 = 100 g
Or 2 M CaCO₃ =   100 �2 = 200 g
(d) 1 M H₂O=     2 � 1+  1 � 16 = 18g
Or 10 M water =  18 �10 = 180g

6. Which of the following has maximum number of atoms?
(a) 18 g of H₂O
(b) 18 g of O₂
(c) 18 g of CO₂
(d) 18 g of CH₄
Ans. (d) 18 g of CH₄
Explanation: Number of atoms


7. Which of the following contains maximum number of molecules?
(a) 1g CO₂
(b) 1g N₂
(c) 1g H₂
(d) 1g CH₄
Ans. (c) 1g H₂

= 3.011 �10 ²³
Molar mass of other molecules are much higher than given mass, so number of molecules in them will be less than that in hydrogen.

Mass of one atom of oxygen is

3.42 g of sucrose are dissolved in 18 g of water in a beaker. The number of oxygen atoms in the solution are

(a) 6.68 x 10²³
(b) 6.09 x 10²²
(c) 6.022 x 10²³
(d) 6.022 x 10²¹

Ans. (a) 6.68 x 10²³

10. A change in the physical state can be brought about
(a) only when energy is given to the system
(b) only when energy is taken out from the system
(c) when energy is either given to, or taken out from the system
(d) without any energy change
Ans. (c) when energy is either given to, or taken out from the system
Explanation: When a solid changes into liquid, it takes energy. When a liquid changes into solid, it releases energy.