8.1.2Additional lessons
Designing in Chemistry — IB Chemistry HL Revision Notes & Topical Questions
5 min read 6 parts Lesson 168 of 172
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 6Variables and Ranges
A sound design gives sufficient relevant data and allows the research question to be answered.
Independent variable
- Independent variable Choose at least five values covering a wide range so that the trend, if any, can be seen clearly.
Dependent variable
- Dependent variable State exactly what will be measured, how and with which instrument, together with its units.
Spacing
- Spacing Values are spread evenly across the range, for example concentrations of 0.10, 0.20, 0.30, 0.40 and 0.50 mol dm⁻³.
Repeats
- Repeats Repeating each condition at least three times allows a mean to be calculated and anomalies to be spotted.
Part 2 of 6Apparatus and Quantities
Apparatus and Quantities
Suitable instruments
- Suitable instruments Choose apparatus with a resolution suited to the size of the quantity measured,
- such as a burette rather than a cylinder for titres.
Quantities
- Quantities Give exact masses, volumes and concentrations that were tested in preliminary trials.
Making solutions
- Making solutions Show how each solution will be prepared, for example by a serial dilution from a stock solution.
Diagram
- Diagram A labelled diagram makes the apparatus arrangement clear and removes ambiguity.
Part 3 of 6Writing the Method
1
List all the materials and apparatus with sizes and concentrations so that another student could repeat the work.
2
Describe in numbered steps how the independent variable is changed and how each reading will be taken.
3
State when measurements are taken, for example the time interval between readings.
4
Explain how the apparatus will be set up and tested before the main run begins.
5
Include safety and disposal instructions at the point where they apply.
Part 4 of 6Dilution Planning
Dilution equation
- Dilution equation Use c₁V₁ = c₂V₂ to find how much stock solution is needed for a diluted sample.
Volumetric flask
- Volumetric flask Add the stock solution to the flask, fill with water to just below the mark, then add dropwise to the mark.
Serial dilution
- Serial dilution Each solution is made by diluting the previous one, which saves chemicals but passes on any error.
Mixing
- Mixing Invert the flask several times to ensure the solution is uniform.
Part 5 of 6Worked Example
Question Describe how to make 100.0 cm³ of 0.050 mol dm⁻³ solution from a stock of 0.50 mol dm⁻³.
- Use c₁V₁ = c₂V₂ so V₁ = 0.050 × 100.0 ÷ 0.50.
- V₁ = 10.0 cm³ of the stock solution.
- Pipette 10.0 cm³ of stock into a 100.0 cm³ volumetric flask and make up to the mark with distilled water.
Answer Dilute 10.0 cm³ of stock to 100.0 cm³
Part 6 of 6Exam Checklist & Tips
8.1.2 – Key Points for the Exam
- Independent variable The variable that the experimenter changes.
- Dependent variable The variable that is measured as the response.
- Stock solution A concentrated solution used to make more dilute ones.
- Serial dilution Making a series of solutions by diluting each one from the previous solution.
- Replicate A repeat of the same measurement under identical conditions.
Common Mistakes
- Choosing only two or three values for the independent variable, which cannot show the pattern reliably.
- Giving a method that is vague, such as 'add some acid', without quantities.
- Using apparatus with poor resolution for the measurement and then ignoring the resulting large uncertainty.
- Leaving out the repeat measurements, so no mean or spread can be found.
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