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3.1.4Unit 3. Practical Skills in Chemistry I

Standard Solutions — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 6 parts Lesson 6 of 10 Unit quiz

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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.
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Section 1 of 6What Is a Standard Solution?

A standard solution has a concentration known accurately.

Definition
  • Definition A standard (volumetric) solution is made by dissolving a precisely weighed mass of solute in a known final volume.
Use
  • Use It is used in titrations as the reference solution against which an unknown concentration is found.
Suitable solids
  • Suitable solids The solid should be pure, stable, soluble and not absorb water from the air, so its mass stays reliable.
Units
  • Units Concentration is quoted in mol dm⁻³, but it can also be expressed in g dm⁻³ or parts per million.

Section 2 of 6Apparatus

Each piece of glassware has a specific job.

Apparatus
Balance
  • Balance A balance reading to three decimal places gives the mass of solute with the smallest practical uncertainty.
Volumetric flask
  • Volumetric flask A volumetric flask has one etched line, so it holds an accurately known volume such as 250.0 cm³ when filled to the mark.
Pipette
  • Pipette A volumetric pipette delivers a fixed volume of the standard solution into a conical flask for titration.
Wash bottle
  • Wash bottle A wash bottle of deionised water rinses solute from the weighing container and funnel into the flask.

Section 3 of 6Preparation Steps

1
Calculate the mass of solute needed, then weigh it accurately into a clean weighing bottle or small beaker.
2
Dissolve the solid in a beaker using a little deionised water, stirring until no solid remains.
3
Pour the solution through a funnel into the volumetric flask and rinse the beaker and funnel into the flask several times.
4
Add deionised water until the level is just below the line, then add the last drops with a dropping pipette until the bottom of the meniscus touches the mark.
5
Stopper the flask and invert it several times so that the concentration is uniform throughout.

Section 4 of 6Reducing Errors

Weighing by difference
  • Weighing by difference Weigh the container with the solid, tip out the solid, weigh the container again, and subtract to find the mass transferred.
Rinsing
  • Rinsing Rinsing all washings into the flask makes sure none of the solute is left behind on glassware.
Meniscus
  • Meniscus Read the meniscus at eye level, so the volume is not misjudged because of parallax.
Temperature
  • Temperature Fill to the line at room temperature because the volume of liquid changes when the solution is warm.

Section 5 of 6Worked Example

Question Calculate the mass of anhydrous sodium carbonate, Na₂CO₃ (Mr = 106.0), needed to prepare 250 cm³ of a 0.100 mol dm⁻³ standard solution.

  1. Convert the volume to dm³250 ÷ 1000 = 0.250 dm³.
  2. n = c × V = 0.100 × 0.250 = 0.0250 mol.
    What the symbols mean: c = concentration (mol dm−3) · n = amount (mol) · V = volume (dm3; divide cm3 by 1000)
  3. Mass = n × Mr = 0.0250 × 106.0 = 2.65 g.

Answer Mass of Na₂CO₃ required = 2.65 g

Section 6 of 6Exam Checklist & Tips

3.1.4 – Key Points for the Exam

  • Standard solution A solution of accurately known concentration.
  • Volumetric flask A flask calibrated to contain one exact volume when filled to its mark.
  • Meniscus
    • The curved surface of a liquid in a narrow container;
      • read the bottom of the curve for colourless solutions.
  • Solute The substance that dissolves in a solvent to make a solution.

Common Mistakes

  • Dissolving the solid directly in the volumetric flask rather than in a beaker, which makes it hard to dissolve and mix properly.
  • Adding water above the mark so that the solution is too dilute.
  • Forgetting to rinse the beaker and funnel, which leaves some solute behind.
  • Calculating the mass using the volume in cm³ without converting to dm³.

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