NCERT Class 9 Science Chapter 3 Atoms and Molecules

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Q1. A .024g sample of compound of oxygen and boron was found by analysis to contain 0.096g of boron and 0.144g of oxygen. calculate the percentage composition of the compound by weight.

Answer:

Total weight of compound = 0.24 g

percentage weight of boron =( 0.096/0.24 )x100 = 40 %

percentage weight of oxygen = (0.144/0.24)x100 = 60%

Q2. When 3.00 g of carbon is burnt in 8.00 g oxygen, 11.00 g of carbon dioxide is produced. What mass of carbon dioxide will be formed when 3.00 g of carbon is burnt in 50.00 g of oxygen? which law of chemical combination will govern your answer?

Answer:

If 3 gms of carbon and 8 gms of oxygen produces 11 gms of carbon dioxide then 3 gms and 50 gms of oxygen would produce 53 gms of carbon dioxide.

It is according to the law of conservation of mass .

Q3. What are polyatomic ions? Give examples

Answer:

Polyatomic ions are the clusters of ions that act as an ion. They carry a fixed charge on them. Example - Ammonium (NH_4)⁺

Q4. Write chemical formulae of the following:

1. Magnesium chloride
2. Calcium oxide
3. Copper Nitrate
4. Aluminium chloride
5. Calcium carbonate

Answer:

1. Magnesium Chloride - MgCl2
2. Calcium Oxide - Cao
3. Copper Nitrate - CuNO3
4. AlCl3
5. CaCO3

Q5. Give the names of the elements present in the following compounds.

1. Quick Lime
2. Hydrogen bromide
3. Baking powder
4. Potassium Sulphate

Answer:

          1. Quick Lime

Chemical formula - CaO
Elements present - Ca, O (Calcium and oxygen)
 
2. Hydrogen bromide
Chemical Formula - HBr
Elements present - H,Br (Hydrogen and Bormide)
 3. Baking powder
Chemical formula - NaHCO3
Elements presents - Na,H,C,O (Sodium, Hydrogen, Carbon and oxygen)

4. Potassium Sulphate
Chemical Formula - K2SO4
Elements present - K,S,O (Potassium , Sulphur, Oxygen)

Q6. Calculate the molar mass of the following substances:

1. Ethyne, C2H2
2. Sulphur molecule, S8
3. Phosphorous molecule, P4, (Atomic mass of phosphorus)
4. Hydrochloric acid, HCl
5. Nitric Acid, HNO3

Answer:

1.  Ethyne, C2H2

Atomic mass of carbon = 12u

atomic mass ofhydrogen = 1u

Therefore molecular mass of C2H2= 12x2 + 1x2 = 26u

2.  Sulphur molecule, S8

Atomic mass of sulphur = 32u

Therefore molecular mass of S8 = 32x 8 = 256u

3.  Phosphorous molecule, P4, (Atomic mass of phosphorus = 31)

Atomic mass of sulphur = 31u

therefore molecular mass of sulphur = 31x4 = 124u

3. Hydrochloric acid, HCl

Atomic mass of hydrogen = 1u

Atomic mass of Cl is = 35.5

therefor molecular mass of HCl = 1 + 35.5 = 36.5u

4.Nitric Acid, HNO3

Atomic mass of hydrogen = 1u

Atmoic mass of Nitrogen = 14u

Atomic mass of Oxygen = 16u

Therefore, molecular mass of HNO3 = 1 + 14 + 16x3 = 63u

 

Q6. What is the mass of :

1. 1 mole of nitrogen
2. 4 moles of aluminium atoms (atomic mass of aluminium=27)
3. 10 moles of sodium sulphite (Na2SO3)?

Answer:

1.  1 mole of nitrogen

Mass = Molar mass x number of moles

here, number of moles = 1

and the Molar mass of Nitrogen = 14g

Therefore, Mass of 1 mole = 14 x 1 = 14g

 2.  4 moles of aluminium atoms (atomic mass of aluminium=27)

Number of moles = 4

Molar mass of Al = 27

Therefore, mass of 4 moles= 27 x 4 = 108g

 3.  10 moles of sodium sulphite (Na2SO3)?

Number of moles = 10

Molar mass of Sodium Sulphite = 2x23 + 32+ 16x3 = 126g

Therefore, mass of 10 moles of Sulphite = 1260g

Q8. Convert into mole:

12g of oxygen gas
20g of water
22g of carbon dioxide

Answer:

1.  12g of oxygen gas

Number of moles = mass/Molar mass

for oxygen,

Molar mass = 32g

and mass of given oxygen = 12g

Therefore, number of moles = 12/32 = 0.375

 2.  20g of water

Molar mass of water (H20) = 1x2 + 16 = 18g

Given mass of water = 20g

Therefore, number of moles = 20/18 = 1.11

 3.  22g of carbon dioxide

Molar mass of carbon dioxide = 44g

Given the mass of carbon dioxide = 22g

therefore, the number of moles = 22/44 = 0.5

Q9. What is the mass of :

1. 0.2 mole of oxygen atoms?
2. 0.5 mole of water molecules?

Answer:

1. 0.2 mole of oxygen atoms?

molar mass of oxygen = 16g

Therefore, mass of 0.2 mole = 16 x 0.2 = 3.2g

 

2. 0.5 mole of water molecules?

Molar mass of water = 18g

Therefore, mass of 0.5 moles of water = 18 x 0.5 = 9g

Q10. Calculate the number of molecules of sulphur S8 present in 16g of solid sulphur.

Answer.

Molar mass of sulphur = 32u

Molar mass of S8 molecule = 32 x 8 = 256g

Number of molecules =( Given mass/Molar Mass)x Avogardo's Number

= (16/256)x(6.023x1023)

= (0.3764)x1023

= 3.764 x1022

Q11. Calculate the number of aluminium ions present in 0.051g of aluminium oxide.

[Hint: The mass of an ion is the same as that of an atom of the same element. Atomic mass of Al = 27u]

Answer: Molecular mass of aluminium oxide (Al2O3)

= 2x27 + 3 x 16

= 102u

therefore, molar mass = 102g

Number of particles in .051g of aluminium oxide

= (Given mass/Molar mass)x Avogardo's number

= (0.051/102)x6.023x1023

= 0.0030 x 1023

Again, each molecule releases double the number of ions , therefore the total number of ions present are = 2 x 0.0030 x 1023

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