20 Case-Based Questions — Atoms and Molecules
Each case below contains a short scenario followed by 2–4 linked questions and clear, exam‑focused answers. These are tailored to NCERT Class 9 Chapter 3 topics and ideal for application-oriented revision.
Riya mixes powdered magnesium and oxygen in a closed container on an accurate balance. After reaction, the balance reading is unchanged.
Two students prepare water samples independently using different sources. Both obtain pure water.
Carbon combines with oxygen to form CO and CO₂ in two different experiments.
A student is asked to find number of oxygen atoms in 2Al₂(SO₄)₃ units.
A compound has empirical formula CH₂ and its molecular mass is found to be 56.
A sample of chlorine shows average atomic mass ≈35.5.
A student writes H₂ + O₂ → H₂O and claims it is balanced.
Compare NaCl (table salt) and CO₂ (carbon dioxide).
Under microscope pollen grains in water appear to move randomly.
An impure sample of NaCl weighing 200 g yields 180 g of pure NaCl after purification.
At 40°C, 40 g of KNO₃ dissolves in 100 g water. A student adds 100 g KNO₃ to 200 g water at 40°C.
Given MgCO₃, a student is asked to find mass percent of magnesium.
In combustion of propane C₃H₈, the balanced equation is C₃H₈ + 5O₂ → 3CO₂ + 4H₂O.
Consider 2Al + 3Cl₂ → 2AlCl₃.
A compound contains 24 g of carbon, 4 g of hydrogen and 32 g of oxygen. Determine empirical formula.
Two samples labelled A and B: A is sodium chloride solution, B is a mixture of sand and salt.
Students debate whether NaCl has molecules.
A student calculates Mr of NaCl using Ar(Cl)=35.45 and Ar(Na)=22.99.
An unknown element X shows 11 protons in nucleus.
A student often loses marks in Mr calculations for not writing Ar values.