This module covers the calculations that sit on top of the mole: how much solute is in a solution, how much gas a reaction makes, how much product you really collect, and why real results differ from the equation. Check the current 0620 syllabus on the Cambridge page for the exact wording you are examined on.
What is this topic really asking you to do?
Every question here follows the same chain: turn what you are given into moles, use the equation ratio, then turn moles back into the quantity asked for. The skill is in the turning, because the given quantity may be cm³, dm³, grams or a volume of gas.
The equation itself has not changed from the previous module. What changes is the unit on each end of the chain.
What should you know first?
You need to balance equations, calculate a relative formula mass and use moles = mass ÷ molar mass. These come from relative masses and amounts and from formulas and equations.
The mole and equation-ratio tutor lets you check a ratio step, and the equation balance reasoning trainer lets you confirm a balanced equation before you start.
Orienting worked example
The numbers below are invented for practice. A student has 25.0 cm³ of sodium hydroxide solution of concentration 0.200 mol/dm³. The equation is NaOH + HCl → NaCl + H₂O. How many moles of HCl are needed to neutralise it exactly?
Step 1, convert the volume: 25.0 cm³ ÷ 1000 = 0.0250 dm³.
Step 2, find moles of NaOH: moles = concentration × volume = 0.200 × 0.0250 = 0.00500 mol.
Step 3, use the ratio: NaOH : HCl is 1 : 1, so 0.00500 mol of HCl is needed.
Notice that the conversion in Step 1 came before anything else. Forgetting it gives an answer 1000 times too large, and a large wrong number still looks plausible.
In what order should you study the lessons?
- Convert solution volume before using concentration. Every solution question starts here, and the conversion is the commonest lost mark.
- Relate gas amount to volume under stated conditions. It reuses the same chain with a gas volume in place of a solution volume.
- Calculate a percentage yield from supplied data. It adds a comparison of actual and predicted mass.
- Distinguish yield from purity. It stops you from mixing up two percentages that look alike.
- Explain an assumption behind a calculation. It turns the numbers into written reasoning, which longer questions reward.
Which traps catch most students?
- Using cm³ where the formula needs dm³.
- Forgetting the equation ratio, for example when two moles of one substance give one mole of another.
- Using a molar volume the question did not state.
- Calling a measured mass a “yield” when the sample was wet or impure.
- Writing “human error” instead of a specific reason that changes the result in a stated direction.
How should you use the practice set?
Work through the mixed practice set only after the lessons. Cover the answers, write every conversion line, and mark method before number. The mistake log and retest queue is a good place to note which step failed so the same error does not return.
Some students follow each lesson yet still lose marks because the working is crowded or the ratio step is skipped. Our teachers watch that working live in online one-to-one Chemistry tuition and correct the step, not just the answer.