H2 Chemistry Unknown Oxidation States: Balance the Electron Amounts

In the 10 November 2025 H2 Chemistry discussion, a student asked how to work out sulfur’s oxidation state in an unfamiliar question. Other participants described using composition and electron accounting. Their recollections are not an official solution, but they point to an important technique: an oxidation-state question may require reacting amounts before the unknown number can be found.

The official 2026 H2 Chemistry 9476 syllabus includes redox explanations using electron transfer or oxidation-number changes, together with constructing redox equations from half-equations. The example below is newly written practice, not a reconstruction of that examination item.

Start with a table of amounts

Suppose 20.0 cm³ of 0.100 mol dm⁻³ acidified potassium manganate(VII) reacts exactly with 100.0 cm³ of 0.100 mol dm⁻³ of an aqueous iron salt. You are told that manganate(VII) becomes Mn²⁺ and the iron ends as Fe³⁺. Find the initial oxidation state of the iron.

Convert volumes into dm³ before using amount = concentration × volume. The manganate(VII) amount is 0.100 × 0.0200 = 0.00200 mol. The amount of iron ions is 0.100 × 0.1000 = 0.0100 mol. Label the species beside each result; two unlabeled mole values are easy to interchange.

Turn the known oxidation-state change into electrons

In MnO₄⁻, oxygen has oxidation state −2, so manganese is +7. The stated product Mn²⁺ has manganese at +2. Each manganese atom therefore gains five electrons. Since each manganate(VII) ion contains one manganese atom, 0.00200 mol of these ions gains 5 × 0.00200 = 0.0100 mol of electrons.

The iron ions must lose that same total amount of electrons under the stated reaction conditions. Dividing 0.0100 mol of electrons by 0.0100 mol of iron ions gives one electron lost per iron ion. An increase of one ending at +3 means that the iron began at +2.

Notice that the volumes alone do not give the electron change. Their concentrations are part of the calculation too. If a later question changes one concentration, recalculate the amounts before comparing them. Keep units visible until the final ratio so that a volume ratio cannot quietly replace a mole ratio.

Check the sign by describing the chemistry

Write “iron loses one electron” beside “oxidation state rises by one”. Then check that +2 becomes +3. If you subtract in the wrong direction and obtain +4 as the starting state, you are describing a decrease to +3: that would be reduction, while manganese is already being reduced.

The electron balance has both a direction and an amount. Checking only that the final number looks familiar can hide an error. Checking only a numerical ratio can also hide a reversed oxidation or reduction statement.

Use half-equations as an independent check

The acidic manganate(VII) half-equation is MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O. For the iron change, write Fe²⁺ → Fe³⁺ + e⁻. Multiply the second by five so that five electrons cancel when the two half-equations are added.

The combined equation is MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 4H₂O + 5Fe³⁺. Count atoms and total charge on both sides. The charge is +17 on each side, and the reacting manganate-to-iron ratio is 1:5, matching 0.00200:0.0100.

Repair the method before doing another paper

On a correction sheet, use four headings: reacting amounts, electrons per known species, total electrons and unknown change per atom or ion. Put each line of working under one heading. If a species contains several atoms of the element whose state changes, include that factor explicitly instead of assuming one atom per ion.

For another original check, keep the manganate data unchanged but replace the iron solution with 50.0 cm³ of 0.100 mol dm⁻³ tin ions, which end as Sn⁴⁺. There are 0.00500 mol of tin ions, so each loses two electrons and begins at +2. This variation tests whether the method responds to changed amounts and a different stated product.

Choose suitable questions from the JC H2 Chemistry collection. Check the syllabus code and question assumptions before using an older paper. Work from stated products and conditions; do not infer the electron change from a memorised reagent ratio alone.