9th Class (CBSE) Science: Atoms and Molecules

Question: What is the mass of 0.2 mole of oxygen atoms?

Answer:

1 mole = 6.022 × 10²³ Oxygen atoms

0.2 mole = 6.022 × 10²³ 2/10

1 mole = 16 g.

0.2 mole =  16 × 0.2

= 3.2 g.

Question: Find the mass of 2 moles nitrogen atoms.

Answer: 1 mole =14 g.

2 moles = 2 × 14

= 28 g

Question: Calculate the mass of 12.044 × 10²⁵ molecules of oxygen (O₂).

Answer: 

GMM of molecule of  O(O₂) 32 g

mass of 6.022 × 10²³ molecules = 32 g

mass of 12.044 × 10²⁵ molecules = 32 /6.022 × 10²³ × 12.044 × 10²⁵

= 32 × 2 × 10²

= 6400 g.

Question: How many moles of CaC0₃ are present in 10 g. of the substance?
(Ca = 40 u; C = 12 u; O = 16 u)

Answer: GMM of CaC0₃ = 40 × 1 + 12 × 1 + 16 × 3
= 40 + 12 +18

= 100 g

100 g CaC0₃ = 1 mole

1 g = 1/100 moles

10 g = 10/100 moles

= 0.1 moles

Question: If 1 mole of nitrogen molecules weighs 28 g , calculate the mass of one molecule of nitrogen in grams.

Answer: 1 mole = 28 g
6.022 × 10²³ = 28 g
1 molecule = 28/6.022 × 10²³
= 4.648 × 10^-23 g

Question: How many moles are these in 34.5 g of sodium? (Atomic mass of Na = 23 u)

Answer:

GAM of Na = 23 g.
23 g. =  1 mole
1 g.  =  1/23 moles
34.5 g.    =  1/23 ×34.5
=  1.5 moles.

Question: Calculate the mass of 3.011 × 10²⁴ g. atoms of carbon.

Answer:

GAM of Carbon = 12 g.

12 =  6.022 × 10²³ atoms

mass of 3.011  × 10²⁴ atoms = 12/ 6.022 × 10²³ × 3.011  × 10²⁴
= 60 g

Question: If 16 grams contains 1 mole atoms, calculate the mass of its single atom.

Answer: 

6.022 × 10²³ = 16 g
1 atom    = 16/ 6.022 × 10²³
= 2.656 × 10^-23

Question: Calculate the number of molecules present in a drop of chloroform (CHCl₃) weghing 0.00239 g.
(Atomic mass: C = 12 u,  H = 1 u, Cl = 35.5 u)

Answer: GMM of CHCl₃ = 12 × 1 + 1 × 1 + 35.5 × 3
= 12 + 1 + 106.5
= 118.5 g
118.5 g. has   = 6.022 × 10²³
0.0239 g has = 6.022 × 10²³/118.5 × 0.0239
= 12.044 × 10¹⁹

Question: What is the mass of 5 moles of sodium carbonate?

Answer:
GMM of Na₂CO₃  = 23 × 2 + 12 ×1 + 16 × 3

= 46 + 12 + 48
= 106 g.
mass of 1 mole Na₂CO₃  = 106 g.

mass of 1 mole  Na₂CO₃  = 106 × 5

=  530 g.

Question: Calculate the mass in grams of 0.17 mole of hydrogen sulphide H₂S.

Answer:

GMM of H₂S = 1 × 2 + 32 × 1
= 2 + 32
= 34 g.
= 1 mole + 34 g.
0.17 moles    = 5.78 g.

Question: Calculate the mole ratio of 240g of calcium and 240g of magnesium. (Ca = 40 u; Mg 24 u)

Answer:

Calcium 40 g = 1 mole
1 g    = 1/40 mole
240 g      =    1/40 × 240
= 6 mole
24 g of Mg    = 1 mole
1 g Mg    =   1/24 moles
240 g     =  1/24 ×240
= 10 mole
So, Ratio ⇒     6:10
⇒      3:5