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

• 1 u (atomic mass unit) or 1 amu is a unit of mass used to express atomic and molecular weights.
• It is specifically defined as 1/12th the mass of a carbon-12 atom.
• This means that if you take a carbon-12 atom and divide its mass by 12, you get 1 u.
• This definition allows for a standardized way to compare the masses of different atoms.

• 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.

The molecular formula of potassium nitrate is KNO₃. Here's why:

• Potassium nitrate consists of potassium (K), nitrogen (N), and oxygen (O).
• Its chemical composition is one potassium atom, one nitrogen atom, and three oxygen atoms.

This combination gives the formula KNO₃.

For the compound to be electrically neutral, the charges of the elements must balance.

Given that:

• X is divalent (charge 2)
• Y is tetravalent (charge 4)

To achieve charge balance:

• Two X ions (2 x 2 = 4)
• One Y ion (1 x 4 = 4)

Thus, the formula for the compound is X₂Y.

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

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.