Concentration units and solution volume
Read concentration labels and keep the amount of solute separate from the volume of solution.
Core and Extended · Core recognises g/dm³ and mol/dm³; mole-based concentration and titration calculations are Extended and follow in separate lessons.
Before you begin
- Recognise that a solution contains a solute and solvent.
- Read a unit as an amount per unit volume.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which part of a salt solution is the solute?
What you will learn
- Interpret g/dm³ and mol/dm³ as concentration units.
- Distinguish concentration, solute amount and solution volume.
A concentration label describes amount per volume
Mass concentration can be expressed in grams per cubic decimetre, g/dm³. Amount concentration can be expressed in moles per cubic decimetre, mol/dm³. Both relate an amount of solute to the final volume of solution, but the numerator uses different measures. The mole is an amount-of-substance unit; the detailed numerical method is Extended.
A label of 5 g/dm³ means five grams of solute per cubic decimetre of solution. It does not mean the entire bottle always contains five grams. A smaller bottle of the same solution contains less total solute while retaining the same concentration. The denominator is final solution volume, rather than a statement that exactly one dm³ of solvent was added.
Keep the volume scale clear
One dm³ is 1000 cm³ and is also one litre. A measuring cylinder labelled in cm³ uses a different scale from a concentration label with dm³ in its denominator. Before a later calculation, the chosen volume unit must match the concentration unit.
Adding water without losing solute increases the solution volume and lowers its concentration. Removing some well-mixed solution takes both solvent and solute with it, so it does not by itself change the concentration of the solution left behind. Use the particle and volume meaning before applying an equation.
Pause and explain
What does a concentration of 8 g/dm³ describe?
Worked example
Two bottles contain the same solution labelled 4 g/dm³. One bottle has twice the volume of the other. Compare concentration and total solute mass.
Show the worked solution
- Both bottles have the same amount per unit volume.
- Therefore their concentrations are equal.
- The bottle with twice the volume contains twice the total solute mass.
Answer Same concentration; twice the volume means twice the solute mass.
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Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Compare what the labels 2 g/dm³ and 2 mol/dm³ measure. Is the number two enough to show equal concentrations in the same measure?
Give me a hint
Read both the numerator and denominator.
Compare my reasoning
- The first is a mass of solute per solution volume.
- The second is an amount in moles per solution volume.
- The measures differ, so the numbers alone do not establish equality in one common measure.
Mass concentration versus amount concentration; equal printed numbers alone do not make them equivalent.
Look for these in your work
- Both numerator measures are identified.
- Units are considered before comparing numbers.
A solution is poured into two differently sized bottles without adding water. Does the smaller bottle have lower concentration? Explain.
Give me a hint
The mixture remains the same solution.
Compare my reasoning
- A well-mixed portion has the same solute proportion as the original solution.
- A smaller bottle contains less solution and less total solute.
- Its amount per unit volume, and therefore concentration, stays the same.
No; it has less total solute but the same concentration.
Look for these in your work
- Total amount is distinguished from concentration.
- No dilution is invented when merely dividing the solution.
Water is added to a salt solution without losing any salt. Predict total solute amount, solution volume and concentration.
Give me a hint
Track what changes in the numerator and denominator.
Compare my reasoning
- The total amount of salt is unchanged.
- The solution volume increases.
- The same solute amount spread through more solution means lower concentration.
Same solute amount, greater solution volume, lower concentration.
Look for these in your work
- The unchanged solute amount is explicit.
- A larger denominator explains the dilution.
Common mistakes
- Treating a concentration label as the total mass in any bottle.
- Using solvent volume where a question specifies final solution volume.
What can you explain now?
Name the numerator quantity in mol/dm³.
Compare with the explanation
Amount of substance in moles
The denominator is solution volume in dm³; grams are the numerator in g/dm³ instead.
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Make it stick
Explain concentration to someone using two bottle sizes. Include both supported units and explain why a bottle's total solute is a different quantity.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.