AQA GCSE Chemistry
AQA GCSE Chemistry 4.3.4 Using Concentrations of Solutions in mol/dm3 Exam Questions
Free AQA GCSE Chemistry exam questions for 4.3.4 Using Concentrations of Solutions in mol/dm3, with a mark scheme for every pack. Covers concentration in g/dm3 and mol/dm3, the titration required practical, and calculating unknown concentrations from titration results.
4.3.4 Using Concentrations of Solutions in mol/dm3 exam questions
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About AQA GCSE Chemistry 4.3.4 Using Concentrations of Solutions in mol/dm3
AQA GCSE Chemistry 4.3.4 deals with how much solute is dissolved in a solution. All students calculate concentration in grams per dm3, while separate Chemistry students also use moles per dm3 and carry out titrations, with titration calculations at Higher tier only. The topic is assessed on Paper 1 and links to reactions of acids in 4.4.2 and the titration required practical. Examiners often ask for unit conversions from cm3 to dm3, a full description of titration technique, identification of concordant results and multi-step calculations of an unknown concentration.
What you need to know
- Concentration in g/dm3 concentration (g/dm3) = mass of solute (g) / volume of solution (dm3). 1 dm3 = 1000 cm3, so divide cm3 by 1000. Higher tier only: explain how the mass of solute and volume of solution are related to concentration.
- Concentration in mol/dm3 concentration (mol/dm3) = moles of solute / volume (dm3). Convert between units: g/dm3 = mol/dm3 x Mr.
- Titration required practical Use a pipette to measure 25.0 cm3 of alkali into a conical flask, add indicator (for example phenolphthalein or methyl orange), and add acid from a burette, swirling, until the indicator just changes colour. Repeat until concordant results (within 0.10 cm3) are obtained.
- Titration calculations (Higher tier only) Find moles of the solution with known concentration, use the equation's mole ratio to find moles of the other reactant, then divide by its volume in dm3 to find its concentration.
- Accurate technique Read the burette at the bottom of the meniscus at eye level, use a white tile to see the colour change, add acid dropwise near the end point, and rinse equipment with the solution it will contain.
Common mistakes in 4.3.4 Using Concentrations of Solutions in mol/dm3
- Forgetting to convert cm3 to dm3 before dividing. Always divide cm3 by 1000.
- Including the rough titre in the mean. Only use concordant results.
- Using a measuring cylinder for the alkali. A pipette is needed for an accurate volume.
- Ignoring the mole ratio, for example in H2SO4 + 2NaOH where 1 mol of acid reacts with 2 mol of alkali.
- Describing the end point as when the solution 'changes colour completely'. It is when one drop causes a permanent colour change.
Try three 4.3.4 Using Concentrations of Solutions in mol/dm3 exam-style questions
1. 4.0 g of sodium hydroxide is dissolved to make 250 cm3 of solution. Calculate the concentration in g/dm3. (2 marks)
Show the mark scheme answer
Volume = 250 / 1000 = 0.250 dm3; concentration = 4.0 / 0.250 = 16 g/dm3.
2. 25.0 cm3 of sodium hydroxide solution is neutralised by 20.0 cm3 of 0.100 mol/dm3 hydrochloric acid. NaOH + HCl → NaCl + H2O. Calculate the concentration of the sodium hydroxide in mol/dm3. (3 marks)
Show the mark scheme answer
Moles HCl = 0.100 x (20.0 / 1000) = 0.00200 mol; 1:1 ratio so moles NaOH = 0.00200 mol; concentration = 0.00200 / 0.0250 = 0.0800 mol/dm3.
3. Describe how a student would carry out a titration to find the volume of acid needed to neutralise 25.0 cm3 of alkali. (4 marks)
Show the mark scheme answer
Use a pipette to transfer 25.0 cm3 of alkali to a conical flask; add a few drops of indicator; add acid from a burette while swirling, dropwise near the end point, until the indicator changes colour permanently; record the volume and repeat until results are concordant (within 0.10 cm3).
4.3.4 Using Concentrations of Solutions in mol/dm3 FAQs
What does AQA GCSE Chemistry 4.3.4 cover?
It covers calculating concentration in g/dm3 and mol/dm3, the titration required practical and using titration results to work out an unknown concentration.
Which paper is 4.3.4 assessed on?
Section 4.3 is assessed on AQA GCSE Chemistry Paper 1. Titration technique can also appear in practical-based questions.
Which parts of 4.3.4 are Higher tier only?
Titration calculations and explaining how mass, volume and concentration are related are Higher tier only. Calculating concentration in g/dm3 and describing the titration method are on both tiers.
