Practical Technique

Question 1

Context: Titrations.

Describe a complete method to prepare a 250cm^3 standard solution of sodium carbonate. [6 marks]
In your answer you should include as much relevant practical detail as possible.

Answer 1

• measure mass of a weighing boat with solid sodium carbonate in it
• tip solid into 100cm^3 beaker, reweigh empty weighing boat and subtract the masses to find added mass
• add distilled water into beaker (<100cm^3) and stir with stirring rod until all solid has dissolved.
• transfer solution into 250cm^3 volumetric flask, using distilled water to wash beaker and stirring rod into flask.
• add distilled water into flask, using a dropper to ensure the bottom of the meniscus is on the graduation mark.
• add stopper, and invert the flask multiple times to ensure a homogenous mixture.

Question 2

Context: Titrations.

A student uses this method to complete a titration.

-Rinse a burette with distilled water
-Fill the burette with sodium hydroxide solution
-Use a measuring cylinder to transfer 25cm^3 of the citric acid solution into a conical flask.
- Add 5cm^3 of indicator
-Slowly, add the sodium hydroxide solution from the burette into the conical flask.
-Add the sodium hydroxide solution dropwise near the end point until the indicator just changes colour.
-Repeat the titration until concordant results are obtained.

The method used by the student includes 4 practical steps that will lead to an inaccurate final result.
Identify these 4 steps, explain why it is a mistake, and suggest how the mistake can be corrected.

Answer 2

Mistake 1: rinsing burette with distilled water will lead to the alkali being diluted when added, which will cause the recorded titre to be larger than it should be. Instead, rinse burette with the alkali.
Mistake 2: using a measuring cylinder, the resolution of which is too large and not precise enough, so instead, use a volumetric pipette and pipette filler.
Mistake 3: Using too much indicator, which is not neutral and will change the pH and therefore the endpoint. Instead, use 2-6 drops of indicator
Mistake 4: Adding the sodium hydroxide solution until the indicator just changes colour. This is a mistake since the flask is not swirled, and so not all the acid may have reacted. Instead, add the alkali until a permanent colour change is observed.

Question 3

Distillation method:
A distillation apparatus consists of a round-bottomed flask, a fractionating column, a thermometer, a condenser and a conical collecting flask.
A student has a mixture of both ethanol and water, from which he wishes to separate the ethanol.
Describe, in full detail, how the student would use his apparatus to collect a sample of ethanol. Use the fact that the boiling point of ethanol is lower than water.

Answer 3

• turn on water in condenser
• heat the flask gently, to produce ethanol and water vapours.
• vapours pass up the fractionating column, and the water is condensed back into water and back into reaction vessel
• ethanol passes through into the condenser, where it is cooled and condenses back into a liquid
• the student should observe the thermometer to ensure the temp is at or below the boiling point of ethanol, so that only ethanol vapours are passing into the condenser.
• one the temp begins to rise above, stop the distillation.