On this page
This chapter is where chemistry stops being descriptive and starts asking you to calculate. The two laws at its start are short to state, yet it is easy to lose marks by treating them as definitions to recite instead of tools to use. This post covers what the 2026–27 syllabus asks, works through each type of calculation, lists the common mistakes and gives you a week's practice plan.
What is in the 2026–27 chapter?
In the new NCERT book *Exploration*, this topic is Chapter 9, Atomic Foundations of Matter. The CBSE Class IX syllabus for 2026–27 lists the law of conservation of mass, the law of constant proportion, Dalton's atomic theory, molecules of elements and covalent compounds, ions and ionic compounds, writing chemical formulae, molecular mass and formula unit mass. The chapter builds on Chapter 8, so read our guide to the structure of the atom in Class 9 first if valency is shaky.
What is not here: the mole. The 2025–26 Class IX syllabus did not list it either. The mole is introduced in Unit 1 of NCERT Class 11 Chemistry. Check the syllabus for your exam year.
How do you use the law of conservation of mass?
Mass is neither created nor destroyed in a chemical reaction, so the total mass of reactants equals the total mass of products. NCERT credits Antoine Lavoisier with the law and tests it by reacting baking soda with vinegar: in an open flask the reading drops because carbon dioxide escapes, but with a balloon sealing the flask the reading stays the same.
Example 1. Heating 10.0 g of calcium carbonate leaves 5.6 g of calcium oxide. How much carbon dioxide was given off? Mass of CO₂ = 10.0 − 5.6 = 4.4 g.
When a question says mass "decreased", look for a gas that escaped. When it "increased", look for a gas from the air that joined in, like oxygen when magnesium burns.
How do you use the law of constant proportions?
A compound always contains its elements in the same ratio by mass, wherever it comes from. Pure water is always hydrogen and oxygen in a 1 : 8 ratio by mass.
Example 2. How much hydrogen and oxygen are in 72 g of water? The ratio 1 : 8 has 9 parts, so hydrogen = 72 × 1/9 = 8 g and oxygen = 72 × 8/9 = 64 g.
Example 3: a limiting amount. Magnesium oxide contains Mg and O in the ratio 24 : 16, that is 3 : 2. If 6 g of magnesium is burned with 6 g of oxygen, how much magnesium oxide forms? 6 g of Mg needs 6 × 2/3 = 4 g of O. So 10 g of MgO forms and 2 g of oxygen is left over. The extra oxygen does not go into the compound, because the ratio is fixed.
How do you write a formula quickly?
NCERT's criss-cross method: write the symbols (cation first for ionic compounds), write the valency or charge under each, cross the numbers over as subscripts, then divide by any common factor.
Aluminium oxide: Al³⁺ and O²⁻ give Al₂O₃. Check: 2 × 3 = 6 positive, 3 × 2 = 6 negative.
Magnesium oxide: Mg²⁺ and O²⁻ give Mg₂O₂, which simplifies to MgO.
Aluminium sulfate: Al³⁺ and SO₄²⁻ give Al₂(SO₄)₃. The bracket shows three whole sulfate ions.
Calcium hydroxide: Ca²⁺ and OH⁻ give Ca(OH)₂, not CaOH₂.
For covalent compounds, the same crossing works with valencies: carbon (4) and chlorine (1) give CCl₄.
How do you calculate molecular and formula unit mass?
Add the atomic masses of every atom in the formula. Covalent compounds form molecules, so the result is a molecular mass. Ionic compounds form crystals, not molecules, so NCERT calls it formula unit mass.
Use H = 1 u, C = 12 u, O = 16 u, Al = 27 u, S = 32 u.
Glucose, C₆H₁₂O₆: (6 × 12) + (12 × 1) + (6 × 16) = 72 + 12 + 96 = 180 u.
Sulfuric acid, H₂SO₄: 2 + 32 + 64 = 98 u.
Aluminium sulfate, Al₂(SO₄)₃: (2 × 27) + 3 × (32 + 4 × 16) = 54 + 3 × 96 = 54 + 288 = 342 u.
Multiply everything inside a bracket by the subscript outside it. That one rule fixes most wrong answers here.
Which mistakes come up most?
Missing brackets: AlOH₃ instead of Al(OH)₃.
Not simplifying: leaving Mg₂O₂ instead of MgO.
Calling an ionic compound a molecule: NaCl has a formula unit, not a molecule.
O versus O₂: an oxygen atom is O; oxygen gas is O₂ molecules.
Thinking an open-flask experiment breaks the law: the missing mass left as gas.
When a definition makes sense in class but not in a question, the Fermi Concept Library covers every concept in chemistry, each with its own lesson, so you can revise valency or ionic bonding on its own before coming back. For the history behind the theory, see our post on John Dalton's atomic theory.
A practice plan for one week
Day 1: Write the formulae for the ion pairs in NCERT's Pause and Ponder questions 17 and 18, checking charges each time.
Day 2: Do three constant-proportion problems, including one with a leftover reactant.
Day 3: Calculate the molecular or formula unit mass of ten compounds from Table 9.1 ions.
Day 4: Explain to someone at home why the open-flask reading dropped.
Start today by writing the formula and formula unit mass of calcium nitrate, Ca(NO₃)₂, and checking it against NCERT's answer of 164 u.
