Understanding Solution Concentration in mol/dm³ Revision Notes | GCSE AQA Higher Chemistry

1. Why Concentration in mol/dm³ Matters

You've probably made squash before - too little concentrate and it tastes weak, too much and it's undrinkable. Chemistry works the same way. When we mix chemicals in reactions, the concentration determines whether the reaction happens fast, slow, or at all.

But here's where chemistry gets more precise than making drinks. We need to know exactly how many particles are dissolved, not just the mass. That's where mol/dm³ comes in - it tells us the number of particles per unit volume.

2. Understanding Concentration in mol/dm³

Think of it this way: if you dissolve sugar in water, the mass tells you how heavy the solution is. But concentration in mol/dm³ tells you how many sugar molecules are swimming around in each litre. This matters because chemical reactions happen between particles, not masses.

**Trap**: Many students think concentration is just 'how strong' something is. At GCSE, concentration in mol/dm³ tells you how many moles of solute are dissolved in each dm³ of solution.

3. The Triangle Relationship

This gives us the triangle relationship: $C = \frac{n}{V}$. Cover up what you want to find, and the triangle shows you the calculation. Need moles? $n = C \times V$. Need volume? $V = \frac{n}{C}$.

But here's the connection to mass that the specification requires. To find moles from mass, we use: $\text{moles} = \frac{\text{mass}}{\text{molar mass}}$. So concentration becomes:

4. Worked Calculations

Let's tackle the type of calculation that appears in every exam. The key is working systematically through the steps, converting units as you go.

Worked example: Finding Concentration from Mass and Volume

Calculate the concentration when 5.85g of sodium chloride (NaCl) is dissolved in 250 cm³ of water. [Molar mass of NaCl = 58.5 g/mol] [4 marks]

1. $V = \frac{250}{1000} = 0.25 \text{ dm³}$ — Always convert cm³ to dm³ by dividing by 1000 [M1]
2. $n = \frac{\text{mass}}{\text{molar mass}} = \frac{5.85}{58.5} = 0.10 \text{ mol}$ — Use the mass and given molar mass [M1]
3. $C = \frac{n}{V} = \frac{0.10}{0.25}$ — Substitute values into C = n/V [M1]
4. $C = 0.40 \text{ mol/dm³}$ — Always include units for the final mark [A1]

Answer: 0.40 mol/dm³

Notice how the molar mass of NaCl (58.5 g/mol) was crucial. Without it, we can't convert from mass to moles. This is why you need to learn common molar masses or be able to calculate them from the periodic table.

Worked example: Finding Mass from Concentration

What mass of potassium hydroxide (KOH) is needed to make 500 cm³ of 0.20 mol/dm³ solution? [Molar mass of KOH = 56.0 g/mol] [3 marks]

1. $V = 0.50 \text{ dm³}$ $n = C \times V$ — Convert cm³ to dm³, then use triangle relationship [M1]
2. $n = 0.20 \times 0.50 = 0.10 \text{ mol}$ — Multiply concentration by volume [M1]
3. $\text{mass} = n \times M_r = 0.10 \times 56.0 = 5.6 \text{ g}$ — Use mass = moles × molar mass [A1]

Answer: 5.6 g

5. Common Exam Traps and How to Avoid Them

Examiners set predictable traps in concentration questions. Here's how to spot them and avoid losing marks.

**The Unit Trap**: Questions often give volume in cm³ but want concentration in mol/dm³. Students who forget to convert get answers that are 1000 times too big. Always write your volume conversion as the first step.

**The Molar Mass Trap**: Some questions give you the mass of solute but not the molar mass. You must calculate it from relative atomic masses in the periodic table. For example, NaCl = 23 + 35.5 = 58.5 g/mol.

Exam precision note: In Understanding Solution Concentration in mol/dm³ - Using concentrations of solutions in mol/dm 3 (chemistry only) (HT only) - Using concentrations of solutions in mol/dm 3 (chemistry only) (HT only), Exam tip: convert between moles per dm3 and particles per dm3 carefully, and explain when an ionic solute produces more than one dissolved particle per formula unit., then link it to evidence, data, or an equation. A high-scoring GCSE answer names the particles involved, states what changes, and justifies the conclusion using correct scientific language. Where calculations are used, show each step with units and keep significant figures sensible. Finally, check that terminology matches AQA wording so your answer is unambiguous and mark-scheme friendly.

At GCSE, focus on the concentration value itself: 1 mol/dm³ means 1 mole of solute dissolved in 1 dm³ of solution. For this topic, you do not need to count the individual dissolved particles.