Cette série est proposée dans sa langue d’origine.

1. Notion: Atomic structure

   1. How many neutrons are in an atom of sodium with mass number 23 and atomic number 11?

   A11B12C23D34
   Correction

   B. 12

   • Neutrons = mass number − atomic number.
   • 23 − 11 = 12.

2. Notion: Atomic structure

   2. What is the electronic configuration of oxygen (atomic number 8)?

   A2, 8B2, 6C2, 4, 2D8
   Correction

   B. 2, 6

   • The first shell holds up to 2 electrons.
   • The remaining 6 go into the second shell: 2, 6.

3. Notion: Periodic table

   3. Which of these is an alkali metal?

   ACalciumBAluminiumCPotassiumDIron
   Correction

   C. Potassium

   • Alkali metals are in Group 1.
   • Potassium is in Group 1; calcium is Group 2, aluminium Group 3 and iron is a transition metal.

4. Notion: Mole concept

   4. What is the molar mass of H₂SO₄? (H = 1, S = 32, O = 16)

   A49 g/molB96 g/molC98 g/molD100 g/mol
   Correction

   C. 98 g/mol

   • (2 × 1) + 32 + (4 × 16).
   • 2 + 32 + 64 = 98 g/mol.

5. Notion: Mole concept

   5. How many moles are in 4 g of sodium hydroxide, NaOH? (Na = 23, O = 16, H = 1)

   A0.1 molB0.4 molC1 molD10 mol
   Correction

   A. 0.1 mol

   • Molar mass of NaOH = 23 + 16 + 1 = 40 g/mol.
   • Moles = mass ÷ molar mass = 4 ÷ 40 = 0.1 mol.

6. Notion: Acids and bases

   6. A solution has a hydrogen ion concentration of 10⁻³ mol/dm³. What is its pH?

   A−3B3C7D11
   Correction

   B. 3

   • pH = −log₁₀[H⁺].
   • −log₁₀(10⁻³) = 3, so the solution is acidic.

7. Notion: Acids and bases

   7. Which gas is produced when zinc reacts with dilute hydrochloric acid?

   AOxygenBChlorineCHydrogenDCarbon dioxide
   Correction

   C. Hydrogen

   • Metal + acid gives a salt and hydrogen.
   • Zn + 2HCl gives ZnCl₂ + H₂.

8. Notion: Organic chemistry

   8. Hydrocarbons with the general formula CₙH₂ₙ₊₂ are called

   AalkenesBalkynesCalkanesDalkanols
   Correction

   C. alkanes

   • Alkanes are saturated hydrocarbons with the formula CₙH₂ₙ₊₂, for example methane, CH₄.
   • Alkenes are CₙH₂ₙ and alkynes are CₙH₂ₙ₋₂.

9. Notion: Redox

   9. In terms of electrons, oxidation is

   Again of electronsBloss of electronsCsharing of electronsDgain of protons
   Correction

   B. loss of electrons

   • Remember OIL RIG: oxidation is loss, reduction is gain of electrons.

10. Notion: Separation techniques

    10. Which method is used to obtain pure water from salt solution?

    AFiltrationBDistillationCChromatographyDDecantation
    Correction

    B. Distillation

    • Distillation boils the water off and condenses the vapour back into pure water.
    • The salt stays behind. Filtration cannot remove dissolved salt.