4.1.1Physical Chemistry
Making a Volumetric Solution — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 6 parts Lesson 95 of 102
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 6Why Standard Solutions Matter
Why Standard Solutions Matter
Volumetric analysis
- Volumetric analysis The volume and known concentration of one solution are used to find the concentration of another.
Standard solution
- Standard solution A solution whose concentration is known accurately, prepared from a precisely weighed mass of solid.
Concentration
- Concentration Concentration is the amount of solute in 1 dm³ of solution, in mol dm⁻³ or g dm⁻³.
Precision
- Precision A volumetric flask and a balance reading to at least two decimal places keep the uncertainty small.
Part 2 of 6Key Apparatus
Balance
- Balance Weighs the solid accurately, ideally to three decimal places, either by difference or by taring.
Beaker
- Beaker Used to dissolve the solid in distilled water before it is transferred.
Glass rod
- Glass rod Stirs the mixture and guides the solution into the flask without spilling.
Volumetric flask
- Volumetric flask Has a single graduation mark, for example 250 cm³, giving a precise volume.
Dropping pipette
- Dropping pipette Adds the last few drops of water so the bottom of the meniscus sits exactly on the mark.
Part 3 of 6Method
1
Weigh the solid accurately, either by difference using a weighing bottle or by taring the beaker on the balance.
2
Dissolve it in distilled or deionised water in a beaker, stirring with a glass rod until all the solid has dissolved.
3
Transfer the solution to the volumetric flask through a funnel, then rinse the beaker, rod and funnel into the flask.
4
Add water until the level is close to the graduation mark, then add the last water drop by drop until the meniscus is on the mark.
5
Stopper the flask and invert it several times so the solution is thoroughly mixed.
Part 4 of 6Avoiding Lost Marks
Avoiding Lost Marks
Name the flask
- Name the flask Say volumetric flask, not just a flask or a beaker, because only it has a precise single mark.
Washings
- Washings Rinse all the apparatus into the flask so that none of the weighed solute is lost.
Water type
- Water type Use distilled or deionised water, since tap water contains dissolved ions that would alter the concentration.
Mixing
- Mixing Invert the stoppered flask at the end, because the solution is not uniform until it is mixed.
Part 5 of 6Worked Example
Question Calculate the mass of anhydrous sodium carbonate, Na₂CO₃ (Mr = 106.0), needed to prepare 250 cm³ of a 0.200 mol dm⁻³ solution.
- Convert the volume250 cm³ = 0.250 dm³.
- Moles = concentration × volume = 0.200 × 0.250 = 0.0500 mol.
- Mass = moles × Mr = 0.0500 × 106.0 = 5.30 g.
Answer 5.30 g of sodium carbonate is dissolved and made up to 250 cm³ in a volumetric flask.
Part 6 of 6Exam Checklist & Tips
4.1.1 – Key Points for the Exam
- Standard solution A solution of accurately known concentration.
- Volumetric flask A flask with one graduation mark that holds a precise volume of solution.
- Meniscus
- The curved surface of a liquid;
- the bottom of it is read against the mark.
- The curved surface of a liquid;
- Concentration The amount of solute per 1 dm³ of solution.
Common Mistakes
- Adding water straight to the mark in the beaker and not using a volumetric flask.
- Forgetting to rinse the beaker and glass rod into the flask.
- Filling the flask above the graduation mark and not starting again.
- Using the volume of water added instead of the final volume of the solution.
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