Q.1
Which of the following are tri-atomic and tetra-atomic molecules?
CH₃Cl, CaCl₂, NH₃, PCl₃, P₂O₅, H₂O, C₂H₅OH
Answer:
(i) Tri-atomic molecules are CaCl₂, H₂O.
(ii) Tetra-atomic molecules are NH₃, PCl₃.

Q.2
Differentiate between the actual mass of a molecule and gram molecular mass.
Answer:
Actual mass of a molecule is obtained by dividing the molar mass by Avogadro’s number whereas gram molecular mass represents the molecular mass expressed in grams, i.e., it is the mass of 1 mole of molecules, i.e., Avogadro’s number of molecules.

Q.3
Calculate the formula mass of sodium carbonate (Na₂CO₃.10H₂O).
Answer:
Formula mass of sodium carbonate
= (2 × atomic mass of Na) + (1 × atomic mass of C) + (3 × atomic mass of O) + 10 [(2 × atomic mass of H) + (1 × atomic mass of O)]
= 2 × 23 + 1 × 12 + 3 × 16 + 10 [(2 × 1) + (1 × 16)]
= 46 + 12 + 48 + 180 = 286 u

Q.4
Calculate the mass of one atom of hydrogen atom.
Answer:
1 mole of hydrogen atom = 1 g
or 6.022 × 10²³ atoms of hydrogen weigh = 1 g
Mass of one atom = 16.022×1023g
= 1.66058 × 10^-24g

Q.5
How many moles are present in 4 g of sodium hydroxide?
Answer:
Gram molar mass of NaOH = 23 + 16 + 1 = 40 g
40 g of NaOH = 1 mol
∴ lg of NaOH = 140mol
∴ 4 g of NaOH = 140 × 4 mol = 0.1 mol

Q.6
A sample of ammonia weighs 3.00 g. What mass of sulphur trioxide contains the same number of molecules as are in 3.00 g ammonia?
Answer:
Number of moles of ammonia in 3.00 g = 3.0017 mol
= 0.1764 mol
Molecular mass of S0₃ = 1 × 32u + 3 × 16u = 80u
1 mole of SO₃ weighs 80 g
∴ 0.1764 moles weigh = 80 × 0.1764 g
= 14.11 g

Q.7

How many (a) molecules (b) hydrogen atoms (c) oxygen atoms are there in 0.5 mol of water?
Answer:
(a) 1 mol of water contains 6.022 × 10²³ molecules
∴ 0.5 mol of water contains 6.022×10232 molecules
= 3.011 × 10²³ molecules

(b) 1 molecule of water contains 2 atoms of hydrogen
1 mol of water contains 2 × 6.022 × 10²³ atoms of hydrogen
∴ 0.5 mol of water contains 2×6.022×10232 atoms of hydrogen
= 6.022 × 10²³ atoms of hydrogen

(c) 1 molecule of water contains 1 atom of oxygen
1 mol of water contains 6.022 × 10²³ atoms of oxygen
∴ 0.5 mol of water contains 6.022×10232 atoms of oxygen
= 3.011 × 10²³ atoms of oxygen

Q.8
How many atoms would be present in a black dot marked on the paper with graphite pencil as a full stop at the end of a sentence. [Given mass of a dot = 10^-18 g]
Answer:
1 mole of carbon atoms weigh = 12 g
Also, 1 mole of carbon atoms = 6.0 2 2 × 10²³ atoms
Thus, 12 g of carbon atoms has 6.022 × 10²³ atoms.
∴ 10^-18 g of carbon will have 6.022×102312×10−18 × 10^-18 carbon atoms
= 5.02 × 10⁴ carbon atoms.

Q 9
Calculate the number of moles present in:
(i) 3.011 × 10²³ number of oxygen atoms.
(it) 60 g of calcium
[Given that atomic mass of Ca = 40 u, Avogadro No. = 6.022 × 10²³]
Answer:
(i) 1 mole of oxygen contains 6.022 × 10²³ atoms
∴ 6.022 × 10²³ atoms of oxygen = 1 mol
1 atom of oxygen = 16.022×1023 mol
∴ 3.011 × 10²³ atoms of oxygen = 1×3.011×10236.022×1023 mol
= 0.5 mol

(ii) Atomic mass of Ca = 40 u
40g of calcium = 1 mol
60g of calcium = 6040 mol =1.5 mol

Q 10
Calculate the mass per cent of each element of sodium chloride in one mole of it.
Answer:
Molecular mass of NaCl = (1 × 23 + 1 × 35.5) u = 58.5 u
Atomic mass of sodium = 23 u

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