Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 2 - 8.23 CORE PRACTICAL 4: Standard solution-Study Notes - New Syllabus

8.23 CORE PRACTICAL 4 – Preparation of a Standard Solution & Titration with NaOH

This practical involves:

• Preparing a standard solution from a solid acid (e.g. oxalic acid)
• Using it to determine the concentration of \( \mathrm{NaOH} \) by titration

(A) Preparation of Standard Solution

Procedure

• Accurately weigh solid acid (e.g. \( \mathrm{H_2C_2O_4 \cdot 2H_2O} \))
• Transfer to a beaker and dissolve in distilled water
• Transfer solution into a volumetric flask
• Rinse beaker and add washings to flask
• Make up to the mark with distilled water
• Stopper and invert several times to mix

Key Equation

\( \mathrm{n = \frac{mass}{M_r}} \), then \( \mathrm{c = \frac{n}{V}} \)

(B) Titration with Sodium Hydroxide 

Procedure

• Pipette 25.0 cm\(^3\) of standard acid into conical flask
• Add phenolphthalein indicator
• Fill burette with \( \mathrm{NaOH} \)
• Titrate until solution turns pale pink
• Record titre and repeat for concordant values

Indicator Choice

• Phenolphthalein: colourless → pink
• Suitable for weak acid–strong base titration

(C) Calculations

• Step 1: Calculate moles of acid
• Step 2: Use mole ratio
• Step 3: Find moles of \( \mathrm{NaOH} \)
• Step 4: Calculate concentration of \( \mathrm{NaOH} \)

Key Skills

• Accurate weighing (analytical balance)
• Quantitative transfer (washing technique)
• Reading meniscus correctly
• Obtaining concordant titres

Sources of Error

• Loss of solid during transfer
• Overshooting endpoint
• Incomplete mixing

Summary

• Prepare standard solution accurately from solid acid.
• Use it to determine unknown \( \mathrm{NaOH} \) concentration.
• Based on stoichiometry and titration technique.

Therefore, this practical combines preparation accuracy with titration analysis.