What are the observations and notes I expect t o make i n the experiment below? Standardisation o f Sodium Hydroxide Stock Solution with Potassium Hydrogen Phthalate Purpose The purpose o f this experiment i s t o standardise a stock solution o f NaOH b y creating a primary standard such a s potassium hydrogen phthalate. You will have t o calculate the exact concentration i n moles per litre o f your NaOH stock solution. The molecular mass o f K H C 8 H 4 O 4 i s 2 0 4 . 2 3 g m o l – 1 .
Answer
Observations and Notes: Standardisation of NaOH using Potassium Hydrogen Phthalate (KHP)
🔍 Materials and Setup Observations
- Potassium hydrogen phthalate (KHP) appears as a white crystalline solid.
- Sodium hydroxide (NaOH) solution is clear and colourless.
- Phenolphthalein indicator is colourless in acidic solution and turns pink in basic solution.
- All glassware must be rinsed and dried properly before use.
⚖️ Weighing and Dissolving KHP
- Accurately weigh a known mass of KHP (e.g., between 0.2 – 0.3 g).
- Dissolve the solid in 50 – 100 mL of distilled water in a conical flask.
- Swirl gently to ensure complete dissolution of KHP before titration.
🧪 Titration Procedure Notes
- Add 2–3 drops of phenolphthalein indicator to the KHP solution.
- Titrate slowly with NaOH from the burette until a pale pink colour persists for 30 seconds.
- Note the initial and final burette readings to calculate the volume of NaOH used.
- Repeat the titration at least 3 times for consistency, ensuring values are within ±0.1 mL.
📄 Calculations and Results
- Moles of KHP = mass of KHP (g) / molar mass (204.23 g/mol)
- Since KHP reacts with NaOH in a 1:1 ratio, moles of NaOH = moles of KHP.
- Calculate concentration of NaOH = moles of NaOH / volume of NaOH used (L)
Use consistent units throughout the calculations. Always convert mL to L when using in molarity formula.
✅ Expected Final Observations
- The appearance of a faint, stable pink colour at the endpoint of titration.
- Consistent volume readings across trials indicating accurate titration technique.
- Calculated molarity of NaOH close to the expected value (e.g., around 0.1 M depending on preparation).
Accurate standardisation is critical for subsequent experiments involving NaOH.