Grade 12 chemistry in Egypt mixes facts to remember (properties, uses, ores) with questions that need a method (electron configurations, moles, masses from equations). Students lose most marks on the second kind, usually through one skipped step. The three examples below are original practice on topics from the book, each with the step most often skipped.
Example 1: electron configuration of Fe²⁺ and Fe³⁺
Iron (atomic number 26): [Ar] 3d⁶ 4s². When iron forms ions, it loses the 4s electrons first. Fe²⁺: [Ar] 3d⁶. Fe³⁺: [Ar] 3d⁵. Fe³⁺ has five unpaired electrons in a half-filled 3d sublevel, which is why it is more stable and strongly paramagnetic.
Common mistake: removing electrons from 3d first, giving Fe²⁺ as [Ar] 3d⁴ 4s². Electrons are removed from the outermost level (4s) before 3d.
Example 2: mass, moles and atoms
How many moles and atoms are in 11.2 g of iron (Fe = 56)? Moles = mass ÷ molar mass = 11.2 ÷ 56 = 0.2 mol. Atoms = moles × Avogadro's number = 0.2 × 6.02 × 10²³ = 1.204 × 10²³ atoms.
Common mistake: multiplying the mass, not the number of moles, by Avogadro's number. Always convert to moles first; the mole is the bridge between mass and particles.
Example 3: mass of iron from reducing iron(III) oxide
In the blast furnace, Fe₂O₃ + 3CO → 2Fe + 3CO₂. What mass of iron comes from 320 g of Fe₂O₃ (Fe = 56, O = 16)? Molar mass of Fe₂O₃ = 2(56) + 3(16) = 160 g/mol, so 320 g = 2 mol. The equation gives 2 mol Fe for every 1 mol Fe₂O₃, so 4 mol Fe = 4 × 56 = 224 g. Check: iron is 112/160 = 70% of the oxide's mass, and 70% of 320 g is 224 g.
Common mistake: using a 1 : 1 ratio. Read the coefficients from the balanced equation before converting back to grams.
In every calculation, go through moles: grams → moles → ratio from the equation → moles → grams or particles.
How EXAMEV helps
The EXAMEV 3rd Secondary Chemistry Test Bank (Egypt) has practice questions lesson by lesson, from transition elements and iron to the mole and chemical analysis, with an explanation for every answer. The examples here are EXAMEV practice, not official exam questions; check the ministry's current specification for this year. For a full-year plan, read our Thanaweya Amma study plan.