(IN TEXT QUESTIONS) (Page 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
Ans: In this reaction, Sodium carbonate reacts with Ethanoic acid to produce Sodium Ethanoate, Carbon dioxide, and 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 = 2.2g (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.
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.
Ans: 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 1g of hydrogen gas is 8g.
Therefore, the mass of oxygen gas required to react completely with 3g of hydrogen gas
is 8 × 3g = 24 g.
3: Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass.
Ans: The postulate of Dalton’s atomic theory which is a result of the law of conservation of mass is “Atoms are indivisible particles, which can neither be created nor destroyed in a chemical reaction”.
4: Which postulate of Dalton’s atomic theory can explain the law of definite proportions.
Ans: The postulate of Dalton’s atomic theory which can explain the law of definite proportion is “The relative number and kind of atoms in a given compound remains constant”.
(IN TEXT QUESTIONS) (Page 35)
1: Define atomic mass unit.
Ans: 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.
Ans: The size of an atom is so small that it is not possible to see it with naked eyes. Also, atom of an element does not exist independently.
(IN TEXT QUESTIONS) (Page 39)
1: Write down the formula of
(i) Sodium oxide (ii) Aluminium chloride
(iii) Sodium sulphide (iv) Magnesium hydroxide
Ans:
(i) Formula of Sodium oxide (ii) Formula of Aluminium chloride
symbol Na O symbol Al Cl
valency 1 2 valency 3 1
Formula Na2O Formula AlCl3
(iii) Formula of Sodium sulphide (iv) Formula of Magnesium hydroxide
symbol Na S symbol Mg (OH)
valency 1 2 valency 2 1
Formula Na2 S Formula Mg (OH)2
2: Write down the names of compounds represented by the following formula:
(i) Al2(SO4)3 (ii) CaCl2 (iii) K2SO4 (iv) KNO3 (v) CaCO3
Ans: (i) Al(SO4)3→ Aluminium sulphate
(ii) CaCl2→ Calcium chloride
(iii) K2SO4→ Potassium sulphate
(iv) CaCO3→ Calcium carbonate.
3: What is meant by the term chemical formula.
Ans: The chemical formula of a compound means the symbolic representation of the composition of a compound. From the chemical formula of a compound, we can know the number and kinds of atoms of different elements that constitute the compound. e.g. from the chemical formula CO2 of carbon dioxide, we come to know that one carbon atom and two oxygen atoms are chemically bonded together to form one molecule of the compound, carbon dioxide.
4: How many atoms are present in a (i) H2S molecule and (ii) (PO4)−3 ion.
Ans: (i) In an H2S molecule, three atoms are present; two of hydrogen and one of sulphur.
(ii) In a (PO4)−3 ion, five atoms are present; one of phosphorus and four of oxygen.
(IN TEXT QUESTIONS) (Page 38)
1: Calculate the molecular masses of H2, O2, Cl2, CO2, CH4, C2H6, C2H4, NH3, and CH3OH.
Ans:
Molecular mass of H2 = 2 × Atomic mass of H
= 2 × 1 = 2 u
Molecular mass of O2 = 2 × Atomic mass of O
= 2 × 16 = 32 u
Molecular mass of Cl2= 2 × Atomic mass of Cl
= 2 × 35.5 = 71 u
Molecular mass of CO2 = Atomic mass of C + 2 × Atomic mass of O
= 12 + 2 × 16 = 44 u
Molecular mass of CH4 = Atomic mass of C + 4 × Atomic mass of H
= 12 + 4 × 1 = 16 u
Molecular mass of C2H6 = 2 × Atomic mass of C + 6 × Atomic mass of H
= 2 × 12 + 6 × 1 = 30 u
Molecular mass of C2H4 = 2 × Atomic mass of C + 4 × Atomic mass of H
= 2 × 12 + 4 × 1 = 28 u
Molecular mass of NH3= Atomic mass of N + 3 × Atomic mass of H
= 14 + 3 × 1 = 17 u
Molecular mass of CH3OH =Atomic mass of C+4 × Atomic mass of H+ Atomic mass of O = 12 + 4 × 1 + 16 = 32 u.
2: Calculate the formula unit masses of ZnO, Na2O, K2CO3, given masses of Zn = 65u, Na = 23u, K= 39u, C = 12u, and O = 16u.
Ans: i) Formula unit mass of ZnO
= Atomic mass of Zn + Atomic mass of O
= 65 + 16
= 81 u
ii) Formula unit mass of Na2O
= 2 × Atomic mass of Na + Atomic mass of O
= 2 × 23 + 16
= 62 u
iii) Formula unit mass of K2CO3
= 2 × Atomic mass of K + Atomic mass of C + 3 × Atomic mass of O
= 2 × 39 + 12 + 3 × 16
= 138 u
(IN TEXT QUESTIONS) (Page 42)
- If one mole of carbon atoms weigh 12 grams, what is the mass (in grams) of 1 atom of carbon.
Ans:
- 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).
Ans:
