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Playbook

Some Basic Concepts

The mole in every form: mass, gas volume, solution concentration, percentage composition and the limiting reagent. Its arithmetic runs through every other physical chapter.

Questions in the bank
205
q/paper in 2025–26
1.50
Calculation
72%
Notes pages
8

Strand: Calculate

When you’ll see it

A mass, a volume of gas, a concentration or a titre to be turned into moles and carried through a formula, an equation or an n-factor.

How this chapter is tested

Almost every question is arithmetic on the mole. Turn what the stem gives into moles, apply a ratio from a formula or a balanced equation, and turn the answer back into grams, litres or a concentration. The bank also files gas laws, oxidation numbers and titrations here, so the chapter is wider than its NCERT name.

A large share are numeric-answer questions, so there is no option to check against and the integer you type has to be exact. The ideas are not hard. Marks go on units, on the molar volume, on water of crystallisation and on n-factors: kelvin in PV = nRT, kilograms of solvent for molality, two H⁺ per H₂SO₄, five electrons for permanganate in acid.

The pages build on each other. Concentration needs the mole; titration needs both concentration and the electron counts from redox balancing. A slip early in the chain, such as an anhydrous molar mass for a hydrate, carries all the way to the answer.

The sub-skills

The distinct skills inside the chapter, in the order to learn them.

  • Measurement and the mole

    n = m/M = N/Nₐ = V/Vₘ; count atoms against molecules; significant figures ignore the power of ten.

  • Percentage composition and formulae

    Masses to moles to the simplest whole-number ratio, then MF = (M ÷ EF mass) × EF; combustion data give C from CO₂ (12/44) and H from H₂O.

  • Stoichiometry and the limiting reagent

    Scale for purity first; the limiting reagent has the smallest n/ν, not the fewest moles; work the product from it.

  • Gas laws and gas volumes

    PV = nRT with T in kelvin and R matched to the units; volumes follow coefficients for gases only; pᵢ = xᵢP.

  • Molarity, dilution and mixing

    M = n/V(L); M₁V₁ = M₂V₂; M_mix = (M₁V₁ + M₂V₂)/(V₁ + V₂); molarity changes with temperature, molality does not.

  • Molality, mole fraction and ppm

    Fix a basis (1 L or 100 g of solution), use the density, and subtract the solute's mass to get the solvent's.

  • Oxidation number and redox balancing

    Sum of oxidation numbers = charge; a fractional value is an average; balance electrons lost = electrons gained, then check the charge.

  • Equivalents and titrations

    M₁n₁V₁ = M₂n₂V₂ at the end point; n is H⁺ or OH⁻ per unit for acid–base and electrons per unit for redox.

Traps to expect

Distractor shapes this chapter reuses. The Traps page covers the ones that cut across chapters.

  • Molecules counted as atoms

    0.1 mol of O₂ holds 0.1Nₐ molecules but 0.2Nₐ atoms. Both values are usually among the options.

  • Fewest moles taken as limiting

    In N₂ + 3H₂, 0.5 mol N₂ and 1.2 mol H₂: H₂ runs out (0.4 < 0.5) though it has more moles. Divide by the coefficient first.

  • Dividing by the solution's mass

    Molality is per kilogram of solvent. Take the solute's mass out of the solution's mass before dividing.

  • The n-factor left at 1

    Ba(OH)₂ and H₂SO₄ carry two; FeC₂O₄ with permanganate carries three (Fe²⁺ gives one, oxalate two). The options sit a factor of two or three apart for this slip.

  • Celsius or the wrong R

    27 °C is 300 K. 0.082 goes with atm and litres, 0.083 with bar and litres, 8.314 with pascals and cubic metres.

Learn it before you drill it

This chapter has full teaching notes — foundations, worked examples, self-checks and a mastery check for each page. Read the notes once, then drill page by page below.

Some Basic Concepts notes

Drill every Some Basic Concepts question

205 questions from the bank, across 8 subtopics.

Drill the 205 questions

Drill one subtopic at a time

The 8 subtopics, in teaching order.

Related playbooks

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