Concentration of SolutionsAQA GCSE Chemistry: Revision notes
Section 1
What is concentration and why does it matter?
Concentration describes how much solute is dissolved in a given volume of solution. It tells us how 'strong' or 'weak' a solution is. In chemistry, we measure concentration in two main ways:
- Moles per cubic decimetre (mol/dm³) — the amount of substance in moles per litre
- Grams per cubic decimetre (g/dm³) — the mass of solute per litre
Concentration is crucial because it affects the rate of reactions, the strength of acids and bases, and the behaviour of solutions in practical work.
Think of concentration like the sweetness of juice: add more sugar powder to the same volume of water and the juice becomes sweeter (higher concentration). Use the same amount of sugar in more water and it tastes less sweet (lower concentration).
Section 2
How do you calculate concentration in mol/dm³?
The formula for calculating concentration is:
Concentration (mol/dm³) = moles ÷ volume (dm³)
This can be rearranged to find moles or volume:
- Moles = concentration × volume
- Volume = moles ÷ concentration
Always ensure the volume is in dm³ (cubic decimetres) before using the formula. Remember that 1 dm³ = 1 litre, so if your volume is given in cm³, you must convert it first.
Examiners expect you to show the formula clearly, substitute numbers with units, and give your final answer with both number and unit. Always include units in every step.
A solution contains 0.5 moles of sodium chloride dissolved in 250 cm³ of water. Calculate the concentration in mol/dm³.
Step 1: Convert 250 cm³ to dm³ by dividing by 1000 → 250 ÷ 1000 = 0.25 dm³
Step 2: Use the formula: concentration = moles ÷ volume = 0.5 ÷ 0.25 = 2.0 mol/dm³
Section 3
How do you convert between cm³ and dm³?
Volume conversions are essential for concentration calculations. The key relationship is:
1 dm³ = 1000 cm³
To convert:
- cm³ to dm³: divide by 1000
- dm³ to cm³: multiply by 1000
| Conversion direction | Operation | Example |
|---|---|---|
| cm³ → dm³ | Divide by 1000 | 500 cm³ = 0.5 dm³ |
| dm³ → cm³ | Multiply by 1000 | 0.25 dm³ = 250 cm³ |
This conversion must be done before using the concentration formula if your volume is given in cm³.
A very common error is forgetting to convert cm³ to dm³, then using the unconverted volume in the formula. This will give an answer 1000 times too large or too small. Always check your volume is in dm³ before calculating concentration.
Section 4
How do you calculate concentration in g/dm³?
You can also express concentration as grams per cubic decimetre (g/dm³). The process is similar to mol/dm³ calculations:
Concentration (g/dm³) = mass of solute (g) ÷ volume (dm³)
This can be rearranged to:
- Mass = concentration × volume
- Volume = mass ÷ concentration
The advantage of g/dm³ is that you can measure mass directly on a balance without needing to calculate moles first. However, to compare solutions chemically, you often need to convert to mol/dm³ using the relative atomic mass (Mᵣ) of the substance.
A solution contains 10.0 g of sodium hydroxide dissolved in 250 cm³ of water. Calculate the concentration in g/dm³.
Step 1: Convert 250 cm³ to dm³ → 250 ÷ 1000 = 0.25 dm³
Step 2: Use the formula: concentration = mass ÷ volume = 10.0 ÷ 0.25 = 40.0 g/dm³
Section 5
How do you convert between g/dm³ and mol/dm³?
To convert between mass and molar concentration, you need the relative atomic mass (Mᵣ) of the solute.
The relationship is:
Moles = mass ÷ Mᵣ
So to convert g/dm³ to mol/dm³:
- Find the molar mass (Mᵣ) of the solute from the periodic table
- Convert mass to moles using moles = mass ÷ Mᵣ
- Use the concentration formula with moles and volume in dm³
Alternatively, if you already have concentration in g/dm³, you can convert directly:
Concentration (mol/dm³) = concentration (g/dm³) ÷ Mᵣ
A solution of sodium chloride (NaCl) has a concentration of 58.5 g/dm³. Convert this to mol/dm³.
Step 1: Calculate Mᵣ of NaCl = 23 + 35.5 = 58.5 g/mol
Step 2: Use the conversion formula: concentration (mol/dm³) = 58.5 ÷ 58.5 = 1.0 mol/dm³
The examiner expects you to show how you calculated Mᵣ by adding up individual atomic masses. Always write out the calculation; don't just state the final mass.
Section 6
How do you use the concentration triangle for calculations?
The concentration triangle is a visual tool that helps you rearrange the formula correctly without having to memorise three versions. The triangle shows:
Moles (or Mass)
/ \
/ \
Volume Concentration
To use the triangle:
- Cover the quantity you want to find
- Use the relationship between the two visible quantities
| If you need to find | Use the relationship |
|---|---|
| Moles | Moles = Concentration × Volume |
| Concentration | Concentration = Moles ÷ Volume |
| Volume | Volume = Moles ÷ Concentration |
This approach ensures you always arrange the formula correctly and reduces the chance of mathematical errors.
The triangle method is especially useful when you've rearranged the formula; it helps you check that your arrangement makes sense. Always state which quantity you are finding before you rearrange.
Must Know
- Concentration is the amount of solute per unit volume of solution, measured in mol/dm³ or g/dm³
- Always convert volume to dm³ before using the concentration formula; divide cm³ by 1000
- Use the formula: concentration (mol/dm³) = moles ÷ volume (dm³), and rearrange as needed
- Convert g/dm³ to mol/dm³ by dividing by the molar mass (Mᵣ) of the solute
- Show all working including formula, substitution with units, and final answer with units — examiners must see your method
- The concentration triangle helps you rearrange the formula correctly: cover the quantity you want to find, and multiply or divide the two visible quantities appropriately
That's the notes covered.
Carry on to the next subtopic.