Build one visible pathway: balanced equation → amount ratio → quantities and units → theoretical value → reasonableness → changed case. A correct final number matters, but it does not show whether the pupil understood the chemical relationships, assumptions or units used to reach it.
The worked example uses idealised values for the formation of magnesium oxide. It focuses on the equation, amount ratio, units and assumptions rather than a practical method.
[1] [2] [3]Equation → amount ratio → unit-bearing conversion
Oxygen is in excess. This is reasoning content, not a practical method.
Supplied valuesAr(Mg) = 24.3 · Ar(O) = 16.0
- 1
Balanced equation
2 Mg(s) + O2(g) → 2 MgO(s)Atoms conserved: 2 Mg atoms · 2 O atoms
- 2
Amount ratio
2 mol Mg : 2 mol MgOn(Mg) : n(MgO) = 1 : 1
- 3
Convert to amount
2.43 g ÷ 24.3 g mol−1= 0.100 mol Mg
- 4
Apply equation ratio
0.100 mol Mg ×1 mol MgO1 mol Mg= 0.100 mol MgO
- 5
Convert to mass
0.100 mol × 40.3 g mol−1= 4.03 g MgO (theoretical)
Read the equation before calculating
Start with this balanced equation:
2 Mg(s) + O₂(g) → 2 MgO(s)
The equation is balanced: two magnesium atoms and two oxygen atoms appear on each side. Its coefficients give an amount ratio (mole ratio) of 2 mol Mg : 1 mol O₂ : 2 mol MgO. Use formula or chemistry content beside a supported closed question to distinguish species, coefficients and subscripts. Praktikal includes chemistry questions, formula blocks and pairing, grouping or ordering interactions.
Ask pupils:
- Which species are represented?
- What does each coefficient count in amount terms?
- Which numbers are part of a formula and must not be changed?
Inspect live answers and progress before moving on. A correct match does not show whether the pupil understands why the equation determines an amount ratio.
Translate coefficients into an amount ratio
Ask pupils to state the stoichiometric amount ratio in the required order – for example, n(Mg) : n(MgO) = 2 : 2 = 1 : 1 – then explain where it came from. A numeric, maths fill-in or closed chemistry item can collect the ratio. Review the reason separately.
Keep three distinctions explicit:
- coefficients are not subscripts;
- an amount ratio is not automatically a mass ratio; and
- the order of the ratio follows the named species.
Praktikal can grade a supported closed or numeric field. It does not automatically diagnose which distinction caused an error.
[2] [3] [4]Annotate one worked conversion
Place the formula, periodic-table information, quantity, units and assumptions beside one worked example. Praktikal has formula, calculator and periodic-table content, tables and bounded questions that can be sequenced together.
Leave enough of the pathway visible for pupils to inspect the chemistry. Ask them to complete only a consequential step and annotate:
- what the quantity represents;
- why that conversion is needed;
- which units enter and leave the step; and
- which equation ratio connects it to the next species.
For one transparent worked path, supply Aᵣ(Mg) = 24.3, Aᵣ(O) = 16.0 and therefore M(MgO) = 40.3 g mol⁻¹. With oxygen explicitly in excess:
n(Mg) = 2.43 g ÷ 24.3 g mol⁻¹ = 0.100 mol
n(MgO) = 0.100 mol × (2 mol MgO ÷ 2 mol Mg) = 0.100 mol
m(MgO) = 0.100 mol × 40.3 g mol⁻¹ = 4.03 g
Every displayed unit cancels or carries through explicitly. These are supplied idealised values for reasoning, not a procedure or a prediction of the mass a practical must produce.
The scaffold is a teaching design, not evidence that Praktikal reduces cognitive load or improves attainment.
[1] [2] [3] [4]Separate the bounded answer from the method
Give a supplied magnesium mass and an explicit oxygen-excess assumption. Ask pupils to calculate the theoretical magnesium oxide mass.
Use automatic grading for the numeric answer after setting the accepted value and tolerance. Collect units and working in a separate short or image response, then review the method.
Praktikal does not automatically balance the equation, convert units, diagnose a method or grade the chemical reasoning. A correct number with unexplained units or assumptions is incomplete evidence.
Compare with a supplied experimental result
Provide the supplied value 3.87 g MgO and ask pupils to compare it with the 4.03 g theoretical value:
percentage yield = (3.87 g ÷ 4.03 g) × 100 = 96.0%
The percentage is mathematically consistent to three significant figures. The value is supplied for reasoning; it is not a report of a classroom experiment, a guarantee of yield or an invitation to infer a practical method.
Use a table for the values, a bounded check for the comparison and an open prompt for assumptions or possible explanations. Teacher review is necessary because chemical plausibility cannot be inferred from the final percentage alone.
Ask:
- Is the result mathematically consistent with the supplied values?
- Which idealising assumptions were used?
- Which explanations are chemically plausible, and what further evidence would be needed?
Change one condition and reconstruct the pathway
Finish by changing the starting quantity. For example, changing the supplied magnesium mass to 4.86 g, with oxygen still in excess, gives 0.200 mol Mg and therefore 8.06 g MgO theoretically. Ask pupils to choose the first relevant step, calculate the result and explain why the pathway stayed the same while the quantities doubled.
Deliver the changed case live, as homework or self-paced work. Inspect answers and progress, give feedback and decide whether to revisit the equation, ratio, units, assumptions or reasonableness check.
One successful calculation is evidence for that task, not proof of durable transfer.
[1] [3] [4]Source notes
Improving Secondary Science: Guidance Report Evidence-informed guidance used for eliciting prior ideas, carefully sequencing models and practical work, and giving structured feedback. It does not evaluate this stoichiometry sequence.
Chemical misconceptions Royal Society of Chemistry teaching material used for the need to connect macroscopic, microscopic and symbolic chemistry and to challenge ideas diagnostically. It does not establish the effectiveness of this particular lesson.
Chemistry Learning Using Multiple Representations: A Systematic Literature Review A review of learning with coordinated text, equations, diagrams, tables, graphs, animation and simulation, including stoichiometry. The included studies are small and heterogeneous, so it does not prove this sequence improves learning.
Abridged Standard Atomic Weights 2024 CIAAW lists standard atomic weights of 24.305 ± 0.002 for magnesium and 15.999 ± 0.001 for oxygen. The worked example states rounded school values of 24.3 and 16.0 and uses supplied idealised quantities, not an expected practical yield.
Teacher feedback
Your 1:1 amount ratio comes from the balanced equation, and each conversion carries its units. Keep the oxygen-excess assumption with the result.