NCERT Class 9th Science
Chapter 3
ATOMS AND MOLECULES
Page 32
Q.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, 0.9g 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
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Q.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 3g of hydrogen gas?
Q.3: Which postulate of Dalton's atomic theory is the result of the law of conservation of mass?
Q.4: Which postulate of Dalton's atomic theory can explain the law of definite proportions?
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Q.1: Define atomic mass unit.
Q.2: Why is it not possible to see an atom with naked eyes?
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Q.1: Write down the formula of
(i) sodium oxide
(ii) aluminium chloride
(iii) sodium suphide
(iv) magnesium hydroxide
Q.2: Write down the names of compounds represented by the following formula: (i) Al2(SO4)3
(ii) CaCl2
(iii) K2SO4
(iv) KNO3
(v) CaCO3
Q.3: What is meant by the term chemical formula?
Q.4: How many atoms are present in a
(i) H_2S molecule and
(ii) PO_4^(3-) ion?
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Q.1: Calculate the molecular masses of H2, O2, Cl2, CO2, CH4, C2H6, C2H4, NH3, CH3OH.
Q.2: Calculate the formula unit masses of ZnO, Na2O, K2CO3, given masses of Zn = 65u, Na = 23u, K = 39u, C = 12u, and O = 16u.
Q.1: If one mole of carbon atoms weighs 12 gram, what is the mass (in gram) of 1 atom of carbon? Ans.1:
Q.2: Which has more number of atoms, 100 grams of sodium or 100 grams of iron (given, atomic mass of
Na = 23u, Fe =56 u)?
Exercises
Q.1: A 0.24 g sample of compound of oxygen and boron was found by analysis to contain 0.096 g if boron and 0.144 g of oxygen. Calculate the percentage composition of the compound by weight.
Q.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 combinations will govern your answer?
Q.3: What are polyatomic ions? Give examples?
Q.4: Write the chemical formula of the following:
(a) Magnesium chloride
(b) Calcium oxide
(c) Copper nitrate
(d) Aluminium chloride
(e) Calcium carbonate
Q.5: Give the names of the elements present in the following compounds:
(a) Quick lime
(b) Hydrogen bromide
(c) Baking powder
(d) Potassium sulphate.
Q.6: Calculate the molar mass of the following substances:
(a) Ethyne, C2H2
(b) Sulphur molecule, S8
(c) Phosphorus molecule, P4 (atomic mass of phosphorus = 31)
(d) Hydrochloric acid, HCl
(e) Nitric acid, HNO3
Q.7: What is the mass of
(a) 1 mole of nitrogen atoms?
(b) 4 mole of aluminium atoms (Atomic mass of aluminium = 27)?
(c) 10 moles of sodium sulphite (Na2SO3)?
Q.8: Convert into mole.
(a) 12g of oxygen gas
(b) 12g of water
(c) 22g of carbon dioxide
Q.9: What is the mass of:
(a) 0.2 mole of oxygen atoms?
(b) 0.5 mole of water molecules?
Q.10: Calculate the number of molecules of sulphur (S8) present in 16g of solid sulphur.
Q.11: 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)
