Chapter 3
Atoms and Molecules
Page- 32
Questions-
1. In a reaction 5.3 g of sodium carbonat reacted with 6 g of acetic acid. The products were 2.2 g of carbon dioxide, 0.9 g water and 8.2 g of sodium acetate. Show that these observation are in agreement with the law of conservation of mass.
Sodium carbonate + acetic acid → sodium acetate + carbon dioxide + water
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2. Hydrogen and oxygen combine in the ratio of 1.8 by mass to farm water. What mass of oxygen gas would be required to react completely with 3 g of hydrogen gas?
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3. Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass?
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4. Which postulate of Dalton’s atomic theory can explain the law of definite proportions?
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Page-35
Questions-
1. Define the atomic mass unit.
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2. Why is it not possible to see an atom with naked eyes?
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Page-39
Questions-
1. Write down the formulae of
(i) sodium oxide
(ii) aluminium chloride
(iii) sodium suphide
(iv) magnesium hydroxide
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2. Write down the names of compounds represented by the following formulae:
(i) Al₂(SO₄)₃
(ii) Cacl₂
(iii) K₂SO₄
(iv) KNO₃
(v) CaCO₃
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3. What is meant by the term chemical formula?
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4. How many atoms are present in a
(i) H₂S molecule and
(ii) PO₄³- ion?
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Page-40
Questions-
1. Calculate the molecular masses of H₂, O₂, Cl₂, CO₂, CH₄, C₂H₆, C₂H₄, NH₃, CH₃OH.
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2. 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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Page-42
Questions-
1. If one mole of carbon atoms weighs 12 grams, what is the mass (in grams) of 1 atom of carbon?
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2. Which has more number of atoms, 100 grams of sodium or 100 grams of iron (given, atomic mass of Na = 23 u, Fe = 56 u)?
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Exercises
1. 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.
2. 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 3.00 g of carbon is burnt in 50.00 g of oxygen? Which law of chemical combination will govern your answer?
3. What are polyatomic ions? Give examples.
4. Write the chemical formulae of the following.
(a) Magnesium chloride
(b) Calcium oxide
(c) Copper nitrate
(d) Aluminium chloride
(e) Calcium carbonate.
5. Give the names of the elements present in the following
compounds.
(a) Quick lime
(b) Hydrogen bromide
(c) Baking powder
(d) Potassium sulphate.
6. Calculate the molar mass of the following substances.