4.1.2Organic Chemistry
Standard Solutions — Edexcel AS Chemistry Revision Notes & Topical Questions
5 min read 6 parts Lesson 34 of 42
Curriculum Equation Key معادلة تفاعل تكوين الإستر (الأسترة) - Esterification Reaction EquationFast Exam Solution
R-COOH + R2-OH ⇌ (conc. H₂SO₄) R-COO-R2 + H₂O
Word Equation & Conditions: حمض كربوكسيلي + كحول (في وجود حمض الكبريتيك المركز كعامل حفاز ونازع للماء) ⇌ إستر + ماء
Location & Specifications: الكيمياء العضوية للثانوية العامة والتحصيلي و Edexcel Chemistry.
Part 1 of 6Volumetric Apparatus
Different glassware is chosen depending on how precisely a volume must be measured.
Volumetric flask
- Volumetric flask Has a single mark on its neck so it holds an exact volume, such as 250.0 cm³, when filled to the line.
Volumetric pipette
- Volumetric pipette Delivers a fixed volume, such as 25.0 cm³, with high precision and is used to measure out samples.
Burette
- Burette Delivers variable volumes and is read to ±0.05 cm³, making it ideal for titrations.
Balance
- Balance A balance reading to three decimal places reduces the percentage error in the mass of solute.
Part 2 of 6Preparing the Solution
A standard solution has a concentration known accurately, so it is made with great care.
1
Calculate the mass of solute needed, then weigh it accurately into a clean weighing boat or small beaker and record the mass.
2
Transfer the solid to a beaker, then rinse the weighing container with deionised water into the beaker so no solid is lost.
3
Add a little deionised water and stir with a glass rod until the solid has dissolved completely.
4
Pour the solution into the volumetric flask through a funnel and rinse the beaker, rod and funnel into the flask.
5
Add water until the level is just below the mark, then add the last drops with a dropping pipette until the bottom of the meniscus is on the line.
6
Stopper the flask and invert it several times so the solution mixes thoroughly and is uniform.
Part 3 of 6Calculations
Calculations
Moles needed
- Moles needed n = concentration (mol dm⁻³) × volume (dm³).
Mass needed
- Mass needed mass = moles × molar mass.
Dilution
- Dilution Moles stay constant on dilution, so c₁V₁ = c₂V₂.
Primary standard
- Primary standard A solid used to make a standard solution must be pure, stable, soluble and not absorb water from the air.
Part 4 of 6Common Problems
Overshooting the mark
- Overshooting the mark Adding too much water dilutes the solution so its concentration is lower than calculated and the batch must be remade.
Losing solute
- Losing solute Failing to rinse the beaker leaves some solute behind so the true concentration is too low.
Not mixing
- Not mixing Without inverting the flask, the top of the solution is more dilute than the bottom.
Choice of solid
- Choice of solid Sodium hydroxide is unsuitable for a standard solution because it absorbs water and carbon dioxide.
Part 5 of 6Worked Example
Question Calculate the mass of anhydrous sodium carbonate, Na₂CO₃ (M = 106.0 g mol⁻¹), needed to make 250.0 cm³ of a 0.100 mol dm⁻³ solution.
- Convert the volume250.0 cm³ = 0.2500 dm³.
- Moles = 0.100 × 0.2500 = 0.0250 mol.
- Mass = 0.0250 × 106.0 = 2.65 g.
Answer 2.65 g of Na₂CO₃
Part 6 of 6Exam Checklist & Tips
4.1.2 – Key Points for the Exam
- Standard solution A solution whose concentration is known accurately.
- Meniscus
- The curved surface of a liquid;
- read at the bottom of the curve for water.
- The curved surface of a liquid;
- Volumetric flask Flask calibrated to contain one exact volume at a stated temperature.
- Primary standard A pure, stable solid of known formula used to make standard solutions.
Common Mistakes
- Adding the solid directly to the volumetric flask rather than dissolving it first in a beaker.
- Filling the flask to the mark before the solid has completely dissolved.
- Rinsing the weighing boat with the wrong volume of water so the total goes above the mark.
- Using 250 cm³ in the calculation instead of 0.250 dm³.
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