required practical exam qs

Question 1

The 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 25 cm3 of the citric acid solution into a
conical flask.
* Add 5 cm3 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 to get concordant results.
The method used by the student includes three practical steps that will lead to an
inaccurate final result.
For each of these three steps
* identify the mistake
* explain why it is a mistake
* suggest how the mistake can be overcome

Answer 1

1) Problem – using a measuring cylinder
b. Explanation – large uncertainty / not
accurate enough
c. Improvement – use a (volumetric)
pipette (Not dropping pipette)
2) Problem – too much indicator
b. Explanation – may react and affect the
endpoint reading
c. Improvement – use a smaller volume
3) Problem – rinsing the burette with
distilled or deionised water
b. Explanation – will slightly dilute the
alkali solution
c. Improvement – rinse the burette with
alkali solution

Question 2

Describe how the students would use this apparatus to collect a sample of ethanol.

Answer 2

• Turn on the water.
• Heat the flask, with a Bunsen burner.
• This causes water and ethanol vapours to be produced.
• Vapours pass up the fractionating column A.
• Water and ethanol are separated in column A.
• Water condenses back into the flask in column A.
• Observe the thermometer at B to keep the temperature
  at or below the boiling point of ethanol.
  Only ethanol vapour (with a little water) passes into the

Question 3

Propanone can be made by reacting propan-2-ol with an excess of
acidified potassium dichromate(VI).
The propanone is removed from the reaction mixture by distillation.
0 3 . 1 Figure 2 shows the apparatus set up by a student to make propanone by this method.
Suitable clamps are used to hold all the apparatus firmly in place.
Figure 2
There are three problems with the apparatus set up in Figure 2.
For each problem:
* identify the problem
* describe the issue it would cause
* suggest how the problem can be solved

Answer 3

1) Anti-bumping granules
1a no anti-bumping granules
1b to create smaller bubbles / to prevent large
bubbles / to prevent mixture jumping into
condenser
2) Open system with no thermometer
2a system should be closed (above flask) to
prevent gases escaping
2b should be closed with (bung +) thermometer
2c to allow collection of propanone (only) =
3) The water direction in the condenser
3a water flows in wrong direction through
condenser
3b condenser not full of water
3c product may not condense

Question 4

A student determines the enthalpy change for the reaction between
calcium carbonate and hydrochloric acid.
CaCO3(s) + 2HCl(aq) → CaCl2(aq) + CO2(g) + H2O(l)
The student follows this method:
* measure out 50 cm3 of 1.00 mol dm–3 aqueous hydrochloric acid using a measuring
cylinder and pour the acid into a 100 cm3 glass beaker
* weigh out 2.50 g of solid calcium carbonate on a watch glass and tip the solid into
the acid
* stir the mixture with a thermometer
* record the maximum temperature reached.
The student uses the data to determine a value for the enthalpy change.
Explain how the experimental method and use of apparatus can be improved to
provide more accurate data.
Describe how this data from the improved method can be used to determine an
accurate value for the temperature change

Answer 4

Use a burette/pipette (instead of a measuring
cylinder)
1b. Use a polystyrene cup (instead of a beaker) /
insulate beaker
1c. Reweigh the watchglass after adding the solid
1d: Use powdered solid
Stage 2: Temperature Measurements
2a. Measure and record the initial temperature of
the solution for a few minutes before addition
2b. Measure and record the temperature after the
addition at regular intervals (eg each minute) for 8+
minutes/until a trend is observed
Stage 3: Temperature Determination
3a. Plot a graph of temperature against time
3b. Extrapolate to the point of addition
3c. Determine ΔT at the point of addition

Question 5

Before adding the solution from the burette in the rough titration, there was an
air bubble below the tap.
At the end of this titration the air bubble was not there.
Explain why this air bubble increases the final burette reading of the rough titration

Answer 5

Solution fills below the tap

Question 6

During the titration the student washed the inside of the conical flask with some
distilled water.
Suggest why this washing does not give an incorrect result

Answer 6

Does not react