A mixed question links a few small skills one after another. If one is weak, the whole chain breaks, even though the question looks like one big problem.
This page helps when single mole questions work but mixed questions do not. For the mass, moles and particles skill in isolation, see converting between mass, moles and particles.
What does it sound like when this happens?
Students describe it in similar words.
- “I can do each type alone but not when they are combined.”
- “I start well and get lost halfway.”
- “My answer is off by a factor of a thousand.”
What is usually behind it?
The usual cause is one skill that is only half secure. It works in a simple question but fails in a longer one, where you are also holding other steps in your head.
A second cause is having no fixed order, so each mixed question is tackled in a different way.
Which five skills should you test?
| Skill | One-line test |
|---|---|
| Relative formula mass | Find M for NaOH (Na 23, O 16, H 1) |
| Moles from mass | How many mol in 4.0 g NaOH? |
| Moles from concentration and volume | How many mol in 50 cm³ of 2.0 mol dm⁻³ solution? |
| Mole ratio from an equation | How many mol of HCl react with 0.10 mol NaOH? |
| Converting back to volume or mass | What volume of 1.0 mol dm⁻³ HCl holds 0.10 mol? |
Answers, in table order:
- M = 23 + 16 + 1 = 40.
- Moles = 4.0 ÷ 40 = 0.10 mol.
- Moles = 2.0 × 50 ÷ 1 000 = 0.10 mol.
- The ratio in NaOH + HCl → NaCl + H₂O is 1 : 1, so 0.10 mol.
- Volume = 0.10 ÷ 1.0 = 0.10 dm³, which is 100 cm³.
Worked example: all five skills in sequence
A question asks: “4.0 g of NaOH is dissolved in water and neutralised with 1.0 mol dm⁻³ HCl. Find the volume of HCl needed.”
- M of NaOH = 40.
- Moles of NaOH = 4.0 ÷ 40 = 0.10 mol.
- Equation ratio is 1 : 1, so moles of HCl = 0.10 mol.
- Volume of HCl = 0.10 ÷ 1.0 = 0.10 dm³.
- Convert: 0.10 × 1 000 = 100 cm³.
Each step is a skill from the table. If you can do the five tests alone, the mixed question is the same five steps.
What can you do this week?
- Do the five one-line tests and mark which fail.
- Give each failed skill two days of short practice on its own.
- Return to a mixed question and write the five steps before calculating.
- Always convert cm³ to dm³ as a separate line.
The mole and stoichiometry steps tool walks through the order, and the lesson on calculating concentration from supplied data drills the concentration step.
When might one-to-one tuition help?
If the tests pass but mixed questions still break, the weak point may be how you choose the next step. A teacher can watch you work and ask which skill you are using on each line.
See online one-to-one Chemistry tuition for how starting works. Record misses in the mistake log so the next repair targets the right skill.