atom and molecules notes class 9 chapter 3
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Page No: 32
1. 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.
Sodium carbonate + ethanoic acid → sodium ethanoate + carbon dioxide + water
Answer
In the given reaction, sodium carbonate reacts with ethanoic acid to produce sodium ethanoate, carbon dioxide, and water.
Sodium Carbonate + Ethanoic acid → sodium ethanoate + carbon dioxide + water
Mass of sodium carbonate = 5.3 g (Given)
Mass of ethanoic acid = 6 g (Given)
Mass of sodium ethanoate = 8.2 g (Given)
Mass of carbon dioxide = 2.2 g (Given)
Mass of water = 0.9 g (Given)
Now, total mass before the reaction = (5.3 + 6) g
= 11.3 g
And, total mass after the reaction = (8.2 + 2.2 + 0.9) g
= 11.3 g
∴ Total mass before the reaction = Total mass after the reaction
Hence, the given observations are in agreement with the law of conservation of mass.
Page No: 33
2. 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?
Answer
It is given that the ratio of hydrogen and oxygen by mass to form water is 1:8.
Then, the mass of oxygen gas required to react completely with 1 g of hydrogen gas is 8 g.
Therefore, the mass of oxygen gas required to react completely with 3 g of hydrogen gas is 8 × 3 g = 24 g.
3. Which postulate of Dalton's atomic theory is the result of the law of conservation of mass?
Answer
Atoms are indivisible particles and can neither be created nor destroyed during any physical or chemical reaction.
4. Which postulate of Dalton's atomic theory can explain the law of definite proportions?
Answer
Atoms of various elements combine in the simple whole number ratio to form compounds.
Page No: 35
1. Define atomic mass unit.
Answer
Mass unit equal to exactly one-twelfth the mass of one atom of carbon-12 is called one atomic mass unit. It is written as 'u'.
2. Why is it not possible to see an atom with naked eyes?
Answer
The size of an atom is so small that it is not possible to see it with naked eyes. Also, the atom of an element does not exist independently.
Page No: 39
1. Write down the formulae of
(i) sodium oxide
Answer
Na2O
(ii) aluminium chloride
Answer
AlCl3
(iii) sodium sulphide
Answer
Na2S
(iv) magnesium hydroxide
Answer
Mg(OH)2
2. Write down the names of compounds represented by the following formulae:
(i) Al2(SO4)3
Answer
Aluminium sulphate
(ii) CaCl2
Answer
Calcium chloride
(iii) K2SO4
Potassium sulphate
(iv) KNO3
Answer
Potassium nitrate
(v) CaCO3
Answer
Calcium carbonate
3. What is meant by the term chemical formula?
Answer
The chemical formula of a compound is a symbolic representation of its composition.
4. How many atoms are present in a
(i) H2S molecule and
(ii) PO43-ion?
Answer
(i) In an H2S molecule, three atoms are present; two of hydrogen and one of sulphur.
(ii) In a PO43-ion, five atoms are present; one of phosphorus and four of oxygen.
Page No: 32
1. 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.
Sodium carbonate + ethanoic acid → sodium ethanoate + carbon dioxide + water
Answer
In the given reaction, sodium carbonate reacts with ethanoic acid to produce sodium ethanoate, carbon dioxide, and water.
Sodium Carbonate + Ethanoic acid → sodium ethanoate + carbon dioxide + water
Mass of sodium carbonate = 5.3 g (Given)
Mass of ethanoic acid = 6 g (Given)
Mass of sodium ethanoate = 8.2 g (Given)
Mass of carbon dioxide = 2.2 g (Given)
Mass of water = 0.9 g (Given)
Now, total mass before the reaction = (5.3 + 6) g
= 11.3 g
And, total mass after the reaction = (8.2 + 2.2 + 0.9) g
= 11.3 g
∴ Total mass before the reaction = Total mass after the reaction
Hence, the given observations are in agreement with the law of conservation of mass.
Page No: 33
2. 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?
Answer
It is given that the ratio of hydrogen and oxygen by mass to form water is 1:8.
Then, the mass of oxygen gas required to react completely with 1 g of hydrogen gas is 8 g.
Therefore, the mass of oxygen gas required to react completely with 3 g of hydrogen gas is 8 × 3 g = 24 g.
3. Which postulate of Dalton's atomic theory is the result of the law of conservation of mass?
Answer
Atoms are indivisible particles and can neither be created nor destroyed during any physical or chemical reaction.
4. Which postulate of Dalton's atomic theory can explain the law of definite proportions?
Answer
Atoms of various elements combine in the simple whole number ratio to form compounds.
Page No: 35
1. Define atomic mass unit.
Answer
Mass unit equal to exactly one-twelfth the mass of one atom of carbon-12 is called one atomic mass unit. It is written as 'u'.
2. Why is it not possible to see an atom with naked eyes?
Answer
The size of an atom is so small that it is not possible to see it with naked eyes. Also, the atom of an element does not exist independently.
Page No: 39
1. Write down the formulae of
(i) sodium oxide
Answer
Na2O
(ii) aluminium chloride
Answer
AlCl3
(iii) sodium sulphide
Answer
Na2S
(iv) magnesium hydroxide
Answer
Mg(OH)2
2. Write down the names of compounds represented by the following formulae:
(i) Al2(SO4)3
Answer
Aluminium sulphate
(ii) CaCl2
Answer
Calcium chloride
(iii) K2SO4
Potassium sulphate
(iv) KNO3
Answer
Potassium nitrate
(v) CaCO3
Answer
Calcium carbonate
3. What is meant by the term chemical formula?
Answer
The chemical formula of a compound is a symbolic representation of its composition.
4. How many atoms are present in a
(i) H2S molecule and
(ii) PO43-ion?
Answer
(i) In an H2S molecule, three atoms are present; two of hydrogen and one of sulphur.
(ii) In a PO43-ion, five atoms are present; one of phosphorus and four of oxygen.
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