CBSE Class 9 Chemistry Atoms and Molecules MCQs Set 03

Answer: (c) 4

Explanation:
1. Mass of Fe in one mole of haemoglobin = 0.33% of 67000 = 221 g.
2. Number of Fe atoms = 221/56 ≈ 4.

Teacher's Note:
1. Iron per mole of haemoglobin divided by 56 gives atoms per molecule.
2. Round to the nearest whole number.

Answer: (d) 3

Explanation:
1. Moles = molarity × volume in litres = 3 × 1 = 3.

Teacher's Note:
1. Molarity is moles of solute per litre of solution.

Answer: (a) 40

Explanation:
1. Molar mass of NaOH = 23 + 16 + 1 = 40 g.
2. % oxygen = 16/40 × 100 = 40%.

Teacher's Note:
1. Divide the mass of the element by the molar mass.
2. Do not forget to multiply by 100.

Answer: (d) 0.10

Explanation:
1. 3BaCl2 + 2Na3PO4 → Ba3(PO4)2 + 6NaCl.
2. 0.5 mol BaCl2 could give 0.5/3 = 0.167 mol; 0.2 mol Na3PO4 could give 0.2/2 = 0.1 mol.
3. Na3PO4 is limiting, so 0.10 mol is formed.

Teacher's Note:
1. The reagent giving less product is the limiting reagent.
2. Use the coefficients 3 and 2.

Answer: (d) CO\( _2 \)

Explanation:
1. 2.24 L at STP = 0.1 mol.
2. Molar mass = 4.4/0.1 = 44 g.
3. CO2 has molar mass 44 g (SO2 = 64, CO = 28, O2 = 32).

Teacher's Note:
1. Molar mass = mass / moles.
2. Compare with the molar masses of the options.

Answer: (a) 16N

Explanation:
1. One O2 molecule has 2 × 8 = 16 electrons.
2. One mole has N molecules, so 16N electrons.

Teacher's Note:
1. Count the electrons in one molecule first.

Answer: (d) Electron

Explanation:
1. The electron has the smallest mass, about 1/1836 of a proton, with the same magnitude of charge.
2. So its charge/mass ratio is the greatest.

Teacher's Note:
1. A neutron has no charge.
2. Compare e/m: electron is about 1836 times that of the proton.

Answer: (a) 50

Explanation:
1. Milli-equivalents of KMnO4 = 50 × 0.1 = 5.
2. Fe2+ loses one electron, so 5 milli-moles of salt reacted.
3. Molar mass of Fe(NH4)2(SO4)2·6H2O = 392, so mass = 0.005 × 392 = 1.96 g.
4. Purity = 1.96/3.92 × 100 = 50%.

Teacher's Note:
1. Milli-equivalents of oxidant equal milli-equivalents of Fe2+.
2. Fe2+ to Fe3+ is a one-electron change.

Answer: (a) \( \frac{14}{17}\times100 \)

Explanation:
1. NH3 has molar mass 14 + 3 = 17 g.
2. Nitrogen contributes 14 g, so % N = (14/17) × 100.

Teacher's Note:
1. Mass percentage = mass of element in one mole / molar mass × 100.
2. Ammonia is NH3, not N2H6.

Answer: (a) 1, 2 and 4

Explanation:
1. Dalton stated that atoms are indivisible, that they combine in simple whole number ratios, and that atoms of different elements have different masses.
2. He also said that all atoms of one element are identical in mass, so statement 3 is not part of his theory.

Teacher's Note:
1. Statement 3 was later changed after isotopes were discovered.

Answer: (c) 85

Explanation:
1. 10 mL of 1 N KMnO4 = 10 milli-equivalents.
2. Equivalent weight of H2O2 = 17, so H2O2 = 10 × 17 = 170 mg = 0.17 g.
3. Purity = 0.17/0.2 × 100 = 85%.

Teacher's Note:
1. Milli-equivalents of oxidant equal milli-equivalents of H2O2.
2. Equivalent weight of H2O2 is 17.

Answer: (a) CH\( _4 \)

Explanation:
1. % H: CH4 = 4/16 = 25%; C2H4 = 4/28 = 14.3%; C6H6 = 6/78 = 7.7%; C2H6 = 6/30 = 20%.
2. Methane has the highest hydrogen percentage.

Teacher's Note:
1. Compare mass of hydrogen to total mass for each compound.
2. The fewer the carbons per hydrogen, the higher the hydrogen percentage.

Answer: (b) 5

Explanation:
1. The hydroxide has two OH groups, so its molar mass = 2 × 150 = 300 g.
2. M(OH)2 weighs 176 + 34 = 210 g.
3. Water of crystallisation = 300 − 210 = 90 g = 5 × 18, so x = 5.

Teacher's Note:
1. Molar mass = equivalent weight × number of OH groups.
2. Subtract the anhydrous mass to get the water.

Answer: (b) Pb, Hg

Explanation:
1. The symbol of lead is Pb (from Latin plumbum).
2. The symbol of mercury is Hg (from Latin hydrargyrum).

Teacher's Note:
1. Several symbols come from Latin names.

Answer: (c) 9.8 g

Explanation:
1. CaCO3 + H2SO4 → CaSO4 + H2O + CO2.
2. 10 g CaCO3 = 0.1 mol, needing 0.1 mol H2SO4.
3. Mass = 0.1 × 98 = 9.8 g.

Teacher's Note:
1. The reaction is one mole to one mole.
2. Molar mass of H2SO4 is 98.

Answer: (a) 0.65 g

Explanation:
1. Zn + H2SO4 → ZnSO4 + H2.
2. 224 mL = 0.01 mol of H2.
3. 0.01 mol Zn = 0.65 g.

Teacher's Note:
1. One mole of Zn gives one mole of H2.
2. 224 mL is 0.01 of 22.4 L.

Answer: (d) protons and neutrons

Explanation:
1. The particles present in the nucleus are called nucleons.
2. They are protons and neutrons.

Teacher's Note:
1. The nucleus holds protons and neutrons only.

Answer: (c) the molecular weight expressed in grams

Explanation:
1. One gram molecule is one mole of a substance.
2. Its weight is the molecular weight expressed in grams.

Teacher's Note:
1. Gram molecule means molar mass in grams.

Answer: (a) 2.33 g

Explanation:
1. Equivalents of H2SO4 = 0.2 × 0.1 = 0.02, which is 0.01 mol (two equivalents per mole).
2. 0.01 mol of BaSO4 = 0.01 × 233 = 2.33 g.

Teacher's Note:
1. Normality = 2 × molarity for sulphuric acid.
2. One mole of H2SO4 gives one mole of BaSO4.

Answer: (a) 14 g

Explanation:
1. CaCO3 → CaO + CO2.
2. 25 g CaCO3 = 0.25 mol gives 0.25 mol CaO = 0.25 × 56 = 14 g.

Teacher's Note:
1. Marble is calcium carbonate.
2. 100 g CaCO3 gives 56 g CaO.

Answer: (c) the equivalent weight of the metal is 4

Explanation:
1. Oxygen combined = 3.15 − 1.05 = 2.10 g.
2. Equivalent weight of metal = mass of metal/mass of oxygen × 8 = 1.05/2.10 × 8 = 4.

Teacher's Note:
1. Equivalent weight is the mass of metal that combines with 8 g of oxygen.
2. The atomic weight cannot be found without the valency.