Molecular Mass MCQs Quiz | Class 9

Class IX Science (Code 086), Unit I: Matter – Its Nature and Behaviour. This quiz covers the concept of Molecular Mass, including its meaning and calculation using atomic masses through various numerical problems. Test your knowledge, view detailed solutions, and submit to download your result PDF.

Understanding Molecular Mass

The molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of the substance. It is therefore the relative mass of a molecule expressed in atomic mass units (u).

For example, to calculate the molecular mass of water (H2O), we add the atomic mass of 2 hydrogen atoms and 1 oxygen atom.

How to Calculate Molecular Mass

Follow these steps to solve numericals:

  • Step 1: Identify the chemical formula of the compound.
  • Step 2: Find the atomic mass of each element present (e.g., H=1u, O=16u, C=12u, N=14u).
  • Step 3: Multiply the atomic mass of each element by the number of atoms of that element in the molecule.
  • Step 4: Add these values together to get the total molecular mass.

Common Examples Table

Compound Formula Calculation Molecular Mass (u)
Water H2O (2 x 1) + (1 x 16) 18 u
Carbon Dioxide CO2 (1 x 12) + (2 x 16) 44 u
Methane CH4 (1 x 12) + (4 x 1) 16 u
Ammonia NH3 (1 x 14) + (3 x 1) 17 u

Formula Unit Mass

The term “formula unit mass” is used for substances whose constituent particles are ions (like Sodium Chloride, NaCl). It is calculated in the same way as molecular mass. For NaCl: (1 x 23) + (1 x 35.5) = 58.5 u.

Extra Practice Questions

  • 1. Calculate the molecular mass of Nitric Acid (HNO3). (Ans: 63u)
  • 2. Find the formula unit mass of Calcium Chloride (CaCl2). (Ans: 111u)
  • 3. Calculate the molecular mass of Ethene (C2H4). (Ans: 28u)
  • 4. Determine the mass of Sulphur molecule (S8). (Ans: 256u)
  • 5. What is the ratio by mass of nitrogen to hydrogen in ammonia? (Ans: 14:3)