Lakhmir Singh & Manjit Kaur Test: Atoms and Molecules - Class 9 MCQ

The molar mass of water (H₂O) is approximately 18 g/mol.
For 360 g of water, the number of moles = 360 g / 18 g/mol = 20 moles.
Number of molecules = 20 moles × 6.022 × 10²³ molecules/mole ≈ 1.2044 × 10²⁵ molecules.

1 u (atomic mass unit) or 1 amu is defined as 1/12th of the mass of a carbon-12 atom.

• Atoms are not always neutral
• they can have charges when they become ions.
• Additionally, atoms can exist independently, especially noble gases
• Therefore, option 'd' is correct

The Law of Definite Proportions states that a compound always contains the same elements in certain fixed proportions by mass, regardless of its source or how it was prepared.

The number of molecules in a sample is determined by Avogadro's number, which is the same for all substances. Hence, for a given mass, the substance with the smallest molar mass will have the most molecules. Among the options, H2 has the smallest molar mass.

Potassium nitrate has the chemical formula KNO₃.

For the compound to be electrically neutral, the charges of the elements must balance. Given X is divalent (charge 2+) and Y is tetravalent (charge 4+), the compound will have the formula X₂Y to balance the charges.

Molecular mass is the sum of the atomic masses of all atoms in a molecule, compared to the mass of a carbon-12 atom.

Calculate the number of moles of oxygen atoms in sucrose and water, then multiply by Avogadro's number to find the total number of oxygen atoms.

Number of moles = Mass (in grams) / Molar mass.

Changing the physical state of matter typically requires energy input or removal to overcome intermolecular forces.

The molar mass of ZnO can be calculated by summing the atomic masses of zinc and oxygen atoms.

• CaCl: Incorrect. Calcium chloride is CaCl₂ because calcium forms a 2+ ion and chloride forms a 1- ion, needing two chloride ions to balance the charge.
• BiPO₄: Correct. Bismuth phosphate is BiPO₄. Bismuth typically forms a 3+ ion, and the phosphate ion is PO₄^3-, balancing the charges.
• NaSO₄: Incorrect. Sodium sulfate is Na₂SO₄. Sodium forms a 1+ ion, so two are needed to balance the 2- charge of sulfate.
• NaS: Incorrect. Sodium sulfide is Na₂S. Sulfur forms a 2- ion, so two sodium ions are needed to balance the charge.

Avogadro's number represents the number of particles (atoms, molecules, ions) in one mole of a substance. Therefore, the number of moles (n) is equal to the number of particles (N) divided by Avogadro's number (No).

Calculate the number of moles of CaCO₃, then use Avogadro's number to find the number of oxygen atoms.