Area of Study 2: How are chemicals measured and analysed?

You will be able to:

• Calculate concentration in different units (mol L⁻¹, g L⁻¹, ppm, % m/v, % v/v).

• Use solubility curves to explain crystallisation and temperature effects.

• Measure and calculate gas volumes using molar volume or the ideal gas law.

• Use volumetric analysis (titrations) to find unknown concentrations.

• Use calibration curves to identify and quantify substances.

1️⃣ Concentration of Solutions

Chemists express concentration in several units:

• Moles per litre (mol L⁻¹)

• Grams per litre (g L⁻¹)

• ppm (mg per litre)

Students learn to convert between these units and prepare accurate solutions using volumetric flasks.

2️⃣ Solubility and Temperature

Solubility curves show how much solute dissolves at different temperatures.

Key ideas:

• Most solids → solubility increases as temperature increases

• Gases → solubility decreases as temperature increases

• Cooling a saturated solution can cause crystallisation

Real-world relevance:

• Rock candy formation

• Ocean oxygen levels

• Environmental analysis of lakes and rivers

3️⃣ Gases and the Ideal Gas Law

Molar Volume at SLC

At 25°C and 100 kPa:

Ideal Gas Law

pV=nRTpV = nRTpV=nRT

Applications:

• Measuring CO₂ produced in reactions

• Analysing gases in breath tests

• Understanding atmospheric pressure changes

4️⃣ Volumetric Analysis (Titration)

Titrations are used to find unknown concentrations by reacting solutions together until the reaction is complete (endpoint).

Steps students learn:

1. Prepare a standard solution

2. Rinse burette and pipette correctly

3. Add indicator (or use pH meter)

4. Perform titration until colour change occurs

5. Repeat until concordant results

6. Perform stoichiometric calculations

Examples:

• Determining acidity of vinegar

• Testing soft drink acidity

• Analysing water hardness

5️⃣ Calibration Curves (Beer–Lambert Law Concept)

Students use a series of known concentrations to generate a standard curve, then use absorbance of an unknown to determine its concentration.

Applications:

• Food colouring analysis

• Water pollution testing (phosphates, nitrates)

• Pharmaceutical concentration measurement

Conceptual Questions

1. Why does solubility of gases decrease as water temperature increases?

2. Why are pH meters more accurate than indicators?

3. What is the purpose of a standard solution?

Calculation Questions

6. Convert 150 ppm nitrate to g L⁻¹.

7. A titration uses 22.60 mL of 0.100 M NaOH. Find the concentration of the HCl solution.

8. Calculate moles using pV=nRTpV = nRTpV=nRT for:

   • p = 100 kPa, V = 2.00 L, T = 298 K

• Forgetting to convert °C → K in gas law calculations.

• Misreading burette values (top vs bottom of meniscus).

• Not rinsing equipment properly.

• Using endpoint instead of equivalence point for explanations

• Memorise the concentration formulas and units.

• Practise titration calculations regularly.

• Write all working clearly to avoid stoichiometry errors.

• Sketch your own sample calibration curves for practice.