Atoms And Molecules
Q38. Give the names of the elements present in the following compounds.
(a) Quick lime
(b) Hydrogen bromide
(c) Baking powder
(d) Potassium sulphate.
Ans. (a) Quick lime or Calcium Oxide (CaO) - Calcium and Oxygen
(b) Hydrogen bromide (HBr) - Hydrogen and Bromine
(c) Baking powder (NaHCO3) – Sodium, Hydrogen, Carbon and Oxygen
(d) Potassium sulphate (K2SO4) – Potassium, Sulphur and Oxygen
Q39. Compute the difference in masses of 103 moles each of magnesium atoms and magnesium ions. (Mass of an electron = 9.1×10–31 kg)
Ans. Mg atom and Mg2+ ion differ by electrons = 2
103 moles of Mg atom and Mg2+ ion differ by electrons = 2 x 103
Mass of 1 electron = 9.1×10–31 kg
Mass of 2 x 103 electron = 2 x 103 x 6.022 x 1023 x 9.1×10–31
= 1.096 x 10-3 kg
Q40. You are provided with a fine white coloured powder which is either sugar or salt. How would you identify it without tasting?
Ans. Upon heating, sugar powder is charred and becomes black while salt does not char. When dissolved in water, salt solution will conduct electricity because it is ionic while sugar solution will not conduct electricity because it is covalent.
Q41. Which postulate of Dalton’s atomic theory can explain the law of definite proportions?
Ans. The last postulate of Dalton’s atomic theory can explain the law of definite proportions.
The last postulate of Dalton’s atomic theory is as follows:
The relative number and kinds of atoms are constant in a given compound.
Q42. State the number of atoms present in each of the following chemical species
Ans. (a) CO32– = 1C + 3O = 4
(b) PO43– = 1P + 4O = 5
(c) P2O5 = 2P + 5O = 7
(d) CO = 1C + 1O = 2
Q43. Which of the following represents a correct chemical formula? Name it.
(a) CaCl (b) BiPO4 (c) NaSO4 (d) NaS
Ans. (b) BiPO4
(a) The correct formula is CaCl2
(c) The correct formula is Na2SO4
(d) The correct formula is Na2S
Q44. Write the cations and anions present (if any) in the following compounds
Q45. Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass?
Ans. Second postulate of Dalton’s atomic theory is the result of the law of conservation of mass.
Second postulate of Dalton’s atomic theory is as follows:
‘Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.”
Q46. Raunak took 5 moles of carbon atoms in a container and Krish also took 5 moles of sodium atoms in another container of same weight. (a) Whose container is heavier? (b) Whose container has more number of atoms?
Ans. (a) Mass of container containing 5 moles of C atoms=5 x 12 = 60 g
Mass of container containing 5 moles of Na atoms=5 x 23 = 115 g
Hence, container of Krish is heavier.
(b) Both containers have same number of atoms since they contain same number of moles.
Q47. Write the chemical formulae of the following.
(a) Magnesium chloride
(b) Calcium oxide
(c) Copper nitrate
(d) Aluminium chloride
(e) Calcium carbonate
Ans. (a) Magnesium chloride – MgCl2
(b) Calcium oxide - CaO
(c) Copper nitrate – Cu(NO3)2
(d) Aluminium chloride – AlCl3
(e) Calcium carbonate – CaCO3
Q48. Which of the following symbols of elements are incorrect? Give their correct symbols
(a) Cobalt CO
(b) Carbon c
(c) Aluminium AL
(d) Helium He
(e) Sodium So
Ans. (a) Cobalt-Co
(b) Carbon – C
(d) Helium – He (correct)
Q49. Classify each of the following on the basis of their atomicity.
(a) F2 (b) NO2 (c) N2O (d) C2H6 (e) P4 (f) H2O2
(g) P4O10 (H) O3 (i) HCl (j) CH4 (k) He (l) Ag
Ans. They are classified as
Monoatomic - (k) He, (l) Ag
Diatomic - (a) F2, (i) HCl
Polyatomic - (b) NO2, (c) N2O, (d) C2H6, (e) P4, (f) H2O2, (g) P4O10, (h) O3, (j) CH4
Q50. What is the SI prefix for each of the following multiples and submultiples of a unit?
(a) 103 (b) 10–1 (c) 10–2 (d) 10–6 (e) 10–9 (f) 10–12
Ans. (a) 103 = kilo
(b) 10–1 = deci
(c) 10–2 = centi
(d) 10–6 = micro
(e) 10–9 = nano
(f) 10–12 = pico
Q51. Write down the names of compounds represented by the following formulae:
Ans. (i) Al2(SO4)3 – Aluminium sulphate
(ii) CaCl2 – Calcium chloride
(iii) K2SO4 – Potassium sulphate
(iv) KNO3 – Potassium nitrate
(v) CaCO3 – Calcium carbonate
Q52. Calcium chloride when dissolved in water dissociates into its ions according to the following equation.
CaCl2 (aq) ‑> Ca2+ (aq) + 2Cl– (aq)
Calculate the number of ions obtained from CaCl2 when 222 g of it is dissolved in water.
Ans. Molar mass of CaCl2 = 40+2x35.5 = 111g
111g of CaCl2 represent = 1 mol
222g of CaCl2 represent = 222/111 x 1 = 2 mol
No. of molecules in 2 mole of CaCl2 = 2 x 6.022 x 1023
1 molecule of CaCl2 form ions = 3
2 x 6.022 x 1023 molecule of CaCl2 form ions = 3 x 2 x 6.022 x 1023
= 3.6132 x 1024
Q54. If one mole of carbon atoms weighs 12 gram, what is the mass (in gram) of 1 atom of carbon?
Ans. 1 mole of carbon atoms weighs 12 gram
1 mole of carbon atoms weighs = 6.022 x 1023
Therefore, 6.022 x 1023 atoms of carbon = 12 g
1 atom of carbon = 12/ (6.022 x 1023)
= 1.993 x 10-23
Q55. 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?
Ans. When 3.0 g of carbon is burnt in 8.00 g oxygen, 11.00 g of carbon dioxide is produced.
When 3.00 g of carbon is burnt in 50.00 g of oxygen, only 8.00 g of oxygen will react to produce 11.00 g of carbon dioxide, rest 42.00 g of oxygen will remain unreacted.
Here, ‘Law of constant proportion’ governs the answer.
Q56. The mass of one steel screw is 4.11g. Find the mass of one mole of these steel screws. Compare this value with the mass of the Earth (5.98 × 1024kg). Which one of the two is heavier and by how many times?
Ans. No. of steel screws in 1 mole = 6.022 x 1023
Mass of 1 steel screw = 4.11g
Mass of 1 mole steel screws = 4.11 x 6.022 x 1023
= 2.475 x 1021 kg
Mass of the earth = 5.98 × 1024 kg
Ratio = Mass of the earth/Mass of the steel screws
= 2.475 x 1021/5.98 × 1024
= 2.4 x 103