Concentration of solutions — AQA GCSE Chemistry
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Concentration of solutions explained
Concentration of solutions covers 2 specification points, including Many chemical reactions take place in solutions.
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and Calculate the mass of solute in a given volume…. Open a point below for its explanation and revision guidance.
Many chemical reactions take place in solutions.
A solution forms when a solute dissolves in a solvent, and many reactions occur between dissolved reactants. The concentration of a solution tells you how much solute is present in a given volume of solution. It can be expressed as mass per unit volume, commonly g dm⁻³, or as mol dm⁻³. For example, dissolving 20 g of sodium hydroxide in water and making the volume up to 1 dm³ gives a concentration of 20 g dm⁻³. Reactions in solution are useful because dissolved particles mix freely and can react quickly. Understanding concentration allows you to compare solutions, predict reaction behaviour and calculate amounts of solute in a given volume.
Students should be able to: calculate the mass of solute in a given volume of solution of known concentration in terms of mass per given volume of solution (HT only) explain how the mass of a solute and the volume of a solution is related to the concentration of the solution. AQA GCSE Chemistry 8462. GCSE e
Concentration links the mass of solute to the volume of solution. For a solution of known concentration in g dm⁻³, mass of solute = concentration × volume in dm³. For example, 250 cm³ of a 40 g dm⁻³ solution contains 40 × 0.250 = 10 g of solute. Rearranging gives concentration = mass ÷ volume and volume = mass ÷ concentration. If concentration is given in mol dm⁻³, first calculate moles using n = c × V, then convert to mass using m = n × Mᵣ. Always convert cm³ to dm³ by dividing by 1000. This relationship allows prediction of how much solute is needed or present in any given volume.
Your focus
- Describe what is meant by a solution and identify solute and solvent.
- State the meaning of concentration and give suitable units.
- Explain why many reactions take place in solution in terms of mixing and collisions.
Show all 6 objectives
- Calculate the mass of solute from concentration and volume.
- Convert between cm³ and dm³ accurately in concentration calculations.
- Explain how changing volume affects concentration for a fixed mass of solute.
Concentration of solutions exam tips
Marking Points
- Define a solution as a mixture formed when a solute dissolves in a solvent.
- State that concentration describes the amount of solute in a given volume of solution.
- Give common units of concentration such as g dm⁻³ or mol dm⁻³.
- Explain that reactions in solution involve dissolved particles that can mix and collide.
- Use the relationship concentration = mass of solute ÷ volume of solution when carrying out simple calculations.
- Use the equation mass of solute = concentration × volume of solution when concentration is in g dm⁻³.
- Convert volume from cm³ to dm³ by dividing by 1000 before substituting.
- Rearrange the relationship to find concentration or volume when the other two quantities are known.
- Convert between moles and mass using n = c × V and m = n × Mᵣ when concentration is in mol dm⁻³.
- Explain that for a fixed mass of solute, increasing the volume decreases the concentration.
Examiner Tips
- 💡Read the unit of concentration carefully and convert volumes to dm³ when needed.
- 💡Write the concentration equation before substituting values to reduce errors.
- 💡Link solution reactions to particle collision ideas when asked to explain rate or reactivity.
- 💡Write down the equation and rearrange it before substituting numbers.
- 💡Convert all volumes to dm³ at the start of the calculation.
- 💡Check that the final answer has the correct unit, such as g or g dm⁻³.
Common Mistakes
- Confusing solute and solvent; the solute is dissolved and the solvent is the liquid that does the dissolving.
- Thinking concentration depends only on the mass of solute; the correct view is that volume of solution also matters.
- Using the volume of solvent instead of the volume of solution; the corrected approach uses the total solution volume.
- Forgetting to convert cm³ to dm³; the corrected method divides the volume in cm³ by 1000.
- Mixing units, such as using cm³ with g dm⁻³; the corrected approach uses consistent units throughout.
- Confusing mass of solute with concentration; the correction is that concentration is mass per unit volume, not the total mass.