Cette série est proposée dans sa langue d’origine.

1. Notion: Mole concept

   1. How many moles are in 22 g of carbon(IV) oxide, CO₂? (C = 12, O = 16)

   A2 molB1 molC0.5 molD0.25 mol
   Correction

   C. 0.5 mol

   • Molar mass of CO₂ = 12 + 2 × 16 = 44 g/mol.
   • Moles = mass ÷ molar mass = 22 ÷ 44 = 0.5 mol.

2. Notion: Atomic structure

   2. What is the electron configuration of sodium (atomic number 11)?

   A1s² 2s² 2p⁷B1s² 2s² 2p⁶ 3s¹C1s² 2s² 2p⁶ 3p¹D1s² 2s⁸ 3s¹
   Correction

   B. 1s² 2s² 2p⁶ 3s¹

   • Fill orbitals in order: 1s holds 2, 2s holds 2, 2p holds 6.
   • That is 10 electrons; the 11th goes into 3s: 1s² 2s² 2p⁶ 3s¹.

3. Notion: Isotopes

   3. Which pair are isotopes?

   A¹²C and ¹⁴CBO₂ and O₃C¹⁴C and ¹⁴NDH₂O and D₂O
   Correction

   A. ¹²C and ¹⁴C

   • Isotopes are atoms of the same element with different numbers of neutrons.
   • ¹²C and ¹⁴C are both carbon (6 protons) with 6 and 8 neutrons.

4. Notion: Chemical bonding

   4. What type of bond holds sodium chloride together?

   ACovalentBMetallicCDativeDIonic (electrovalent)
   Correction

   D. Ionic (electrovalent)

   • Sodium gives one electron to chlorine, forming Na⁺ and Cl⁻ ions.
   • The attraction between oppositely charged ions is an ionic bond.

5. Notion: Acids and bases

   5. Find the pH of 0.01 mol/dm³ hydrochloric acid.

   A1B0.01C2D12
   Correction

   C. 2

   • HCl is a strong acid, so [H⁺] = 0.01 = 10⁻² mol/dm³.
   • pH = −log[H⁺] = 2.

6. Notion: Gas volumes

   6. What volume do 2 moles of oxygen occupy at s.t.p.? (Molar volume = 22.4 dm³)

   A22.4 dm³B44.8 dm³C11.2 dm³D67.2 dm³
   Correction

   B. 44.8 dm³

   • Volume = moles × molar volume.
   • = 2 × 22.4 = 44.8 dm³.

7. Notion: Redox

   7. What is the oxidation number of manganese in KMnO₄?

   A+7B+5C+6D+2
   Correction

   A. +7

   • K is +1 and each O is −2, and the compound is neutral.
   • 1 + Mn + 4(−2) = 0, so Mn = +7.

8. Notion: Tests for gases

   8. Which gas turns limewater milky?

   AOxygenBHydrogenCAmmoniaDCarbon(IV) oxide
   Correction

   D. Carbon(IV) oxide

   • CO₂ reacts with limewater, Ca(OH)₂, to form insoluble calcium carbonate.
   • The white precipitate makes the limewater milky.

9. Notion: Chemical equilibrium

   9. For N₂ + 3H₂ ⇌ 2NH₃ (exothermic), what happens to the yield of ammonia when the pressure is increased?

   AIt decreasesBIt stays the sameCIt increasesDThe reaction stops
   Correction

   C. It increases

   • The left side has 4 moles of gas and the right side has 2.
   • Higher pressure favours the side with fewer gas moles, so more NH₃ forms.

10. Notion: Organic chemistry

    10. What is the IUPAC name of CH₃CH₂OH?

    AMethanolBEthanolCEthanoic acidDEthane
    Correction

    B. Ethanol

    • Two carbon atoms gives the stem eth-.
    • The −OH group makes it an alkanol, so the name ends in -ol: ethanol.

11. Notion: Organic chemistry

    11. Which functional group is found in alkanoic acids?

    A−COOHB−OHC−CHOD−COO−
    Correction

    A. −COOH

    • Alkanoic acids such as ethanoic acid contain the carboxyl group.
    • The carboxyl group is −COOH.

12. Notion: Water

    12. Which type of hardness can be removed by boiling?

    APermanent hardness, caused by calcium sulphateBPermanent hardness, caused by magnesium chlorideCAll hardnessDTemporary hardness, caused by calcium hydrogencarbonate
    Correction

    D. Temporary hardness, caused by calcium hydrogencarbonate

    • Boiling decomposes calcium hydrogencarbonate: Ca(HCO₃)₂ → CaCO₃ + H₂O + CO₂.
    • The calcium ions leave as insoluble CaCO₃, so temporary hardness is removed.

13. Notion: Electrolysis

    13. How many faradays are needed to deposit 1 mole of copper from Cu²⁺ ions?

    A1B0.5C2D4
    Correction

    C. 2

    • Cu²⁺ + 2e⁻ → Cu.
    • Each mole of copper needs 2 moles of electrons, which is 2 faradays.

14. Notion: Empirical formula

    14. A compound contains 40% carbon, 6.7% hydrogen and 53.3% oxygen by mass. Find its empirical formula. (C = 12, H = 1, O = 16)

    ACHOBCH₂OCC₂H₄ODCH₃O
    Correction

    B. CH₂O

    • Divide by the relative atomic masses: C 40 ÷ 12 = 3.33, H 6.7 ÷ 1 = 6.7, O 53.3 ÷ 16 = 3.33.
    • Divide by the smallest (3.33): C 1, H 2, O 1.
    • Empirical formula: CH₂O.

15. Notion: Reaction rates

    15. How does a catalyst increase the rate of a reaction?

    AIt lowers the activation energyBIt raises the temperatureCIt increases the concentrationDIt changes the products
    Correction

    A. It lowers the activation energy

    • A catalyst provides another reaction pathway with a lower activation energy.
    • More particles have enough energy to react, so the rate increases. The catalyst is not used up.