Atoms Molecules Exemplar — Class 9 Science NCERT Solutions (Free)
Free step-by-step NCERT solutions for Class 9 Science chapter "Atoms Molecules Exemplar" — 5 important questions with detailed answers for CBSE board exam preparation.
TL;DR: Free step-by-step NCERT solutions for Class 9 Science chapter "Atoms Molecules Exemplar" — 5 important questions with detailed answers for CBSE board…
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NCERT Solutions for Class 9 Science — Atoms Molecules Exemplar. Step-by-step answers to all 5 textbook questions from this chapter, written for CBSE board preparation and free to use.
Atoms Molecules Exemplar — All 5 Questions Solved
Q1. Calculate the molar mass of CaCO3 (Ca = 40, C = 12, O = 16).
Molar mass of CaCO3 = 1(40) + 1(12) + 3(16) = 40 + 12 + 48 = 100 g/mol.
Q2. How many atoms of oxygen are present in 3.2 g of SO2? (S = 32, O = 16, NA = 6.022 × 10^23)
Molar mass of SO2 = 32 + 2(16) = 64 g/mol. Moles of SO2 = 3.2/64 = 0.05 mol. One molecule of SO2 has 2 oxygen atoms. Number of O atoms = 0.05 × 2 × 6.022 × 10^23 = 6.022 × 10^22.
Q3. What is the law of constant proportions? Give an example.
The law states that a chemical compound always contains the same elements in the same proportion by mass. Example: water always contains hydrogen and oxygen in mass ratio 1:8 (2g H to 16g O). No matter the source of water, this ratio remains constant.
Q4. How many molecules are present in 88 g of CO2? (C = 12, O = 16, NA = 6.022 × 10^23)
Molar mass of CO2 = 12 + 2(16) = 44 g/mol. Moles = 88/44 = 2 mol. Number of molecules = 2 × 6.022 × 10^23 = 1.204 × 10^24.
Q5. Define relative atomic mass and molecular mass.
Relative atomic mass: mass of an atom compared to 1/12 of mass of carbon-12 atom. Example: Na = 23 (sodium atom is 23 times heavier than 1/12 of C-12). Relative molecular mass: sum of relative atomic masses of all atoms in a molecule. Example: H2O = 1 + 1 + 16 = 18.
How to Use These Solutions
Attempt each question yourself first and only then compare with the worked answer. Marks in the CBSE board exam are awarded for the METHOD as much as the final result, so reproduce the steps rather than memorising the last line. Where a solution states a law, a formula or a definition, learn that wording — examiners look for it.
Related practice for this chapter: chapter MCQs, previous-year questions, revision notes and sample papers.