Assignment - Atom and Molecule, Class 9, Science PDF Download

Important Points

While doing numerical problems of mole concept, the following relations should be remembered.

Key for symbols

Number of moles = n
Given mass = m
Molar mass = M
Given number of particle = N
Avogadro number of particle = N₀

Key Formula

1 mole of atoms = 6. 022 × 10²³ atoms = Gram atomic mass or Molar mass of element

Number of moles =  Mass of element / Molar Mass

Number of moles = Given number of atoms / Avogadro number

or m = n × M and N = n × N₀

Let us apply these formula to solve numerical problems. 

Q. In a reaction 5.3 g of sodium carbonate reacted with 6 g of ethanoic acid. The products were 2.2 g of carbon dioxide. 0.9 g water and 8.2 g of sodium ethanoate. Show that these observations are in agreement with the law of conservation of mass.

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Sodium carbonate + ethanoic acid  sodium ethanoate + carbon dioxide + water.

Q. 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 3 g of hydrogen gas?

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

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(b) Law of constant Proportions / Law of definite proportions :

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

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Q. Define the atomic mass unit.

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

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Q. Calculate the molecular masses of H₂, O₂, Cl₂, CO₂, CH₄, C₂H₆, C₂H₄, NH₃, CH₃OH.

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

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Q. Calculate the molar mass of the following substances.

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1. Ethyne, C₂H₂
2. Sulphur molecule, S₈
3. Phosphorous molecule, P₄(Atomic mass of phosphorus = 31)
4. Hydrochloric acid, HCl
5. Nitric acid, HNO₃

Q. _​What is meant by the term chemical formula?

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Q.  How many atoms are present in a

(i) H₂S molecule and

(ii)   ?

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Q. What are polyatomic ions? Give examples

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

1. Quick lime
2. Hydrogen bromide
3. Baking powder
4. Potassium sulphate

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Q. Write down the formulae of

1. sodium oxide
2. aluminium chloride
3. sodium sulphide
4. magnesium hydroxide

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Q. Write down the names of compounds represented by the following formulae :

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1. Al₂(SO₄)₃
2. CaCl₂
3. K₂SO₄
4. KNO₃
5. CaCO₃

Q. Write the chemical formulae of the following.

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

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Q. If one mole of carbon atoms weighs 12 grams, what is the mass (in grams) of 1 atom of carbon?

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Q. Which has more number of atoms, 100 grams of sodium or 100 grams of iron (given, atomic mass of
Na = 23u, Fe = 56u)?

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Q. A 0.24 g sample of compound of oxygen and boron was found by analysis to contain 0.096 g of boron and 0.144 g of oxygen. Calculate the percentage composition of the compound by weight.

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Q. When 3.0 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 3.0 g of carbon is burnt in 50.00 g of oxygen? Which law of chemical combination will govern your answer?

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Q. What is the mass of –
(a) 1 mole of nitrogen atoms?
(b) 4 moles of aluminium atoms (Atomic mass of aluminium = 27)?
(c) 10 moles of sodium sulphite (Na₂SO₃)?

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Q. Convert into mole.
(a) 12 g of oxygen gas

(b) 20 g of water

(c) 22 g of carbon dioxide

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Q. What is the mass of :
(a) 0.2 mole of oxygen atoms?

(b) 0.5 mole of water molecules?

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