CBSE Class 9 Chemistry Atoms and Molecules MCQs Set 04

Answer: (c) oxalate

Explanation:
1. The C2O4 2− ion is oxalate.
2. Sodium oxalate is Na2C2O4.

Teacher's Note:
1. Carbonate is CO3 2−; formate is HCOO−; acetate is CH3COO−.

Answer: (b) one-half mole of H\( _2 \)

Explanation:
1. Mg + 2H+ → Mg2+ + H2.
2. 12 g Mg = 0.5 mol, which gives 0.5 mol of H2.

Teacher's Note:
1. One mole of Mg gives one mole of H2.
2. 12 g is half a mole of magnesium.

Answer: (c) 2 g

Explanation:
1. One mole of H2 molecules weighs 2 g.
2. So Avogadro's number of hydrogen molecules weighs 2 g.

Teacher's Note:
1. H2 has molar mass 2 g.

Answer: (d) 8 moles

Explanation:
1. 4 moles of NH3 contain 4 × 4 = 16 gram atoms.
2. Each mole of CO has 2 gram atoms, so 16/2 = 8 moles are needed.

Teacher's Note:
1. NH3 has 4 atoms per molecule; CO has 2.

Answer: (a) the number of atoms present in one molecule

Explanation:
1. A chemical formula shows the elements present and the number of atoms of each in one molecule or formula unit.

Teacher's Note:
1. The subscripts give the number of atoms.

Answer: (a) the atomic weight expressed in grams

Explanation:
1. One gram atom is one mole of atoms.
2. Its weight is the atomic weight expressed in grams.

Teacher's Note:
1. Gram atom means atomic mass in grams.

Answer: (d) SO\( _2 \), SO\( _3 \)

Explanation:
1. The law applies to two compounds formed by the same two elements.
2. SO2 and SO3 are both made of sulphur and oxygen, with oxygen masses in the ratio 2 : 3 for a fixed mass of sulphur.

Teacher's Note:
1. Look for the same pair of elements in both compounds.

Answer: (c) 25 g

Explanation:
1. 16 g dissolves in 100 g of water.
2. 4 g needs 100 × 4/16 = 25 g of water.

Teacher's Note:
1. Set up a direct proportion between solute and water.
2. Divide first, then multiply.

Answer: (b) 2.16 g

Explanation:
1. Ag2CO3 → 2Ag + CO2 + ½O2 on strong heating.
2. 2.76 g Ag2CO3 = 2.76/276 = 0.01 mol, which gives 0.02 mol of Ag.
3. Residue = 0.02 × 108 = 2.16 g.

Teacher's Note:
1. On strong heating silver carbonate leaves metallic silver.
2. Ag2CO3 has molar mass 276.

Answer: (c) (NH\( _4 \))\( _3 \)[Fe(CN)\( _6 \)]

Explanation:
1. Ferricyanide is [Fe(CN)6]3−, with charge 3−.
2. Three ammonium ions (NH4)+ balance it, giving (NH4)3[Fe(CN)6].

Teacher's Note:
1. Balance the charges of the cation and the anion.
2. The ammonium ion is NH4+.

Answer: (a) 50 mL increase

Explanation:
1. 4PH3 → P4 (solid) + 6H2.
2. 4 volumes of PH3 give 6 volumes of H2 (phosphorus is solid).
3. 100 mL PH3 gives 150 mL H2, so the volume increases by 50 mL.

Teacher's Note:
1. Solid products do not count in gas volumes.
2. Use the gas volume ratio from the balanced equation.

Answer: (c) −273°C

Explanation:
1. Kinetic energy is proportional to the absolute temperature.
2. It becomes zero at 0 K, which is about −273°C.

Teacher's Note:
1. Absolute zero is 0 K = −273°C.

Answer: (d) an atom of a compound

Explanation:
1. A compound is made of molecules or formula units, which in turn contain atoms of the elements.
2. An atom belongs to an element, so 'an atom of a compound' is incorrect.

Teacher's Note:
1. Atoms belong to elements; molecules belong to elements or compounds.

Answer: (a) 0.5 g

Explanation:
1. 6 g Mg = 0.25 mol, giving 0.25 mol H2.
2. Mass of H2 = 0.25 × 2 = 0.5 g.

Teacher's Note:
1. Moles of Mg equal moles of H2.
2. Convert moles of H2 to grams at the end.

Answer: (c) H\( _2 \)O < H\( _2 \)S < CO\( _2 \) < SO\( _2 \)

Explanation:
1. Molecular weights: H2O = 18, H2S = 34, CO2 = 44, SO2 = 64.
2. The increasing order is H2O < H2S < CO2 < SO2.

Teacher's Note:
1. Compute each molecular weight first.
2. Check the direction of the signs.

Answer: (c) low temperature and high pressure

Explanation:
1. Lowering the temperature reduces kinetic energy of molecules.
2. High pressure brings the molecules close together so that attractions hold them as a liquid.
3. A catalyst has no role.

Teacher's Note:
1. Cooling and compressing liquefy a gas.

Answer: (d) sulphite

Explanation:
1. Ammonium (NH4+), sodium (Na+) and bicarbonate (HCO3−) all carry a single charge.
2. Sulphite (SO3 2−) carries two charges, so it is divalent.

Teacher's Note:
1. Univalent means a charge of one.

Answer: (d) 82 g

Explanation:
1. CaO + 2HNO3 → Ca(NO3)2 + H2O.
2. 56 g CaO = 1 mol; 63 g HNO3 = 1 mol.
3. HNO3 is limiting: 1 mol gives 0.5 mol Ca(NO3)2 = 0.5 × 164 = 82 g.

Teacher's Note:
1. Find the limiting reagent before calculating the product.
2. Two moles of HNO3 are needed per mole of CaO.

Answer: (b) 14 g

Explanation:
1. 100 g CaCO3 gives 56 g CaO.
2. 25 g gives 25 × 56/100 = 14 g.

Teacher's Note:
1. Use the mass ratio 100 : 56.
2. One quarter of 100 g gives one quarter of 56 g.

Answer: (c) law of conservation of mass

Explanation:
1. A balanced equation has equal numbers of each kind of atom on both sides.
2. This supports the law of conservation of mass.

Teacher's Note:
1. Balancing is done to keep mass the same.