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SSC CGL Chemistry · Mole Concept and Stoichiometry

The Mole and the Laws of Combination

Avogadro's number, the percentage of an element in a compound, balancing an equation, and the laws of chemical combination with the people and years behind them.

All Mole Concept and Stoichiometry notes

Why this matters

Seven questions in the bank: two from the 2023 shifts and five from the 2024 shifts. Three are short sums (a percentage or a balanced equation) that the one formula on this page answers.

Concept 1 of 2: Percentage composition and balancing

Add up the masses of all the atoms to get the molar mass. An element's share is its own mass over that total.

Definition

The steps:

  • Molar mass: add the atomic masses (H 1, C 12, O 16, S 32).
  • Percentage of an element = its mass in one formula / molar mass x 100. The percentages add to 100.
  • Avogadro's constant: 6.022 x 10^23 particles in a mole.
  • Balancing: match carbon, then hydrogen, then oxygen. Burning ethanol, C2H5OH + 3O2 gives 2CO2 + 3H2O.

Percentage composition

% element=mass of element in formulamolar mass×100\%\ \text{element} = \frac{\text{mass of element in formula}}{\text{molar mass}} \times 100
  • molar massSum of the atomic masses in one formula

Worked example

Find the percentage of sulphur in H2SO3.
Practise this conceptself-check · 3 quick reps

The same idea in a real exam question:

SSC CGL · 2024 · Tier I, 17 Sep 2024, Shift 2 · Q33Easy

Example 1 · Mole Concept and Stoichiometry · The Mole and the Laws of Combination

The chemical formula of glucose is C6H12O6\text{C}_6\text{H}_{12}\text{O}_6. The weight % of carbon in glucose is:

Check the percentages add to 100

In a composition question the options often list the right numbers in the wrong order. Match each to its element, then check they add to 100.
Drill 3 more on percentage composition and balancing

Concept 2 of 2: The laws and their discoverers

Each law of chemical combination has one name attached. SSC asks the name or the year.

Definition

The laws:

  • Conservation of mass: Lavoisier, set out in his 1789 Elementary Treatise. He also published the Method of Chemical Nomenclature in 1787, the basis of today's compound names.
  • Definite proportions: Proust. Multiple proportions: Dalton.
  • Avogadro (1811): equal volumes of gases at the same temperature and pressure hold equal numbers of molecules. This reconciled Dalton's atoms with Gay-Lussac's combining volumes.
Law or workWho and when
Conservation of massLavoisier
Method of Chemical NomenclatureLavoisier, 1787Q
Equal volumes, equal moleculesAvogadro, 1811Q
Definite proportionsProust
Multiple proportionsDalton
Practise this conceptself-check · 3 quick reps

The same idea in a real exam question:

SSC CGL · 2024 · Tier I, 12 Sep 2024, Shift 3 · Q26Easy

Example 2 · Mole Concept and Stoichiometry · The Mole and the Laws of Combination

What was Antoine-Laurent Lavoisier's most important contribution to chemistry in 1789?

Mass is Lavoisier, energy is physics

Lavoisier's law is conservation of mass. Conservation of energy belongs to physics and is the decoy.
Drill 2 more on the laws and their discoverers

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

  • Percentage composition and balancing

    Percentage composition

    % element=mass of element in formulamolar mass×100\%\ \text{element} = \frac{\text{mass of element in formula}}{\text{molar mass}} \times 100

Reference tables (1)

The laws and their discoverers5 rows
Law or workWho and when
Conservation of massLavoisier
Method of Chemical NomenclatureLavoisier, 1787Q
Equal volumes, equal moleculesAvogadro, 1811Q
Definite proportionsProust
Multiple proportionsDalton

Watch out for (2)

Test yourself on a real paper

Sit a past SSC CGL paper, timed and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.

Sit an SSC CGL past paperDrill all 7 questions