PRACTICAL GUIDE

Question 1

6 marker standard solutiom

Answer 1

Weigh sample bottle conatining solid on a balance
Transfer sample to beaker and reqeigh sample bottle
Record difference in mass
Add distilled water
Stir
Until solid dissolved
Transfer to volumetric flask using funnel
With washing
Make upto 250cm3
Mix

Question 2

Percentage error

Answer 2

(Eror in experiment x 100)/ amount measured

Question 3

Why is it important to remove any air bubbles from the burette tap

Answer 3

Volume of air bubble would be counted in the burette reading

Question 4

If bubble was not rmoved from the burette tap , how would this effect the titre

Answer 4

Increase it

Question 5

Why is it important to remove funnel used to fill up the burette before starting a funnel

Answer 5

Solution drops could fall from the funnel intothe burette and affect the burette reading

Question 6

Why is it good practice to swirl the clonical flask and rinse the walls of the clonical flask with distillated water during titration

Answer 6

Encures all reactants mix and no reagents is left untreated on sides of flask

Question 7

Why does adding water to the conical flask not affect the titre?

Answer 7

It doesn’t change the number of moles in the conical flask

Question 8

Why is a conical flask is preferred to a beaker for a titration?

Answer 8

There is less chance of splashing liquid while swirling

Question 9

What should the burette be washed with before a titration?

Answer 9

Whatever solution it is going to be filled with

Question 10

Why is it bad practice to wash the burette with water before a titration?

Answer 10

Water would decrease the concentration of the solution in the burette, which would increase the titre

Question 11

9. What should the pipette used to fill up the conical flask be washed with?

Answer 11

Whatever solution is going to be put in the conical flask

Question 12

10.Why is it bad practice to wash the pipette with water before using it to fill up the
conical flask?

Answer 12

Water would take up space in pipette and decrease the number of moles
transferred to the conical flask, which would reduce the volume needed from the
burette (titre) for complete reaction

Question 13

6 marker measurement of an enthalpy change

Answer 13

Record the temperature for a suitable time (3 minutes) before adding reactants together
• To establish an accurate initial temperature
• Mix reactants then record temperature every minute until a trend is seen
• Plot a graph of temperature against time
• Extrapolate the cooling curve back to the point of addition
• To establish a theoretical temperature change accounting for heat loss

Question 14

The initial rate of the reaction between sodium thiosulfate and hydrochloric acid
can be monitored by measuring the time taken for a fixed amount of sulfur
to be produced.
Na2S2O3(aq) + 2 HCl(aq) → 2 NaCl(aq) + SO2 (g) + S(s) + H2O(l)
Describe an experiment to investigate the effect of temperature on the
initial rate of this reaction.
Include
• a brief outline of your method
• how you will measure the time taken for a fixed amount of sulfur to
be formed
• how you will present your results in graphical form
• a sketch of the graph that you would expect.

Answer 14

Stage 1 Method
(1a) Idea of using disappearing cross or colorimetry
(1b) Puts acid into container on cross or in
colorimeter
(1c) Add second reactant and start timing
Stage 2 Measurements
(2a) Repeat at different temperatures
(2b) Record time, t, for cross to disappear / defined reading on colorimeter
(2c) ensure other variables (cross, volumes, concentrations) kept constant
Stage 3 Use of Results
(3a) 1/t is a measure of rate
(3b) plot of rate against T
(3c) sketch of plot as shown

Question 15

Initial Rates
A(aq) + B(aq) + C(aq) → D(aq) + E(aq)
In aqueous solution, A, B, C and D are all colourless but E is dark blue.
A reagent (X) is available that reacts rapidly with E. This means that, if a small amount of X is included in the initial reaction mixture, it will react with any E produced until all of the X has been used up.
Explain, giving brief experimental details, how you could use a series of experiments to determine the order of this reaction with respect to A. In each experiment you should obtain a measure of the initial rate of reaction.

Answer 15

Stage 1 Preparation
1a Measure (suitable/known volumes of) A, B and C
1b Use of colorimeter
1c into separate container(s)
Stage 2 Procedure
2a Start clock/timer at the point of mixing
2b Take series of colorimeter readings at regular time intervals
2c Use of same concentration of B and C / same total volume / (same
volume/amount of X)
2d Same temperature
2e Repeat with different concentrations of A
Stage 3 Use of Results
3a Plot absorbance vs time and measure/calculate gradient at time=0
3b plot of gradient against volumes/concentrations of A
3c description of interpreting order from shape of 1/time vs volume or concentration graph

Question 16

Continuous Monitoring
Potential 6 marker:
Describe how you can determine the order with respect to NaOH in the hydrolysis of methylethanoate ester [6]
CH3COOCH3 + NaOH → CH3COONa + CH3OH

Answer 16

Stage 1: Method
1a- Add 50cm3 of 1 moldm-3 CH3COOCH3 to a 250cm3 beaker + 50cm3 of 1 moldm-3
NaOH, start the timer.
1b – Every 30 seconds, transfer 5cm3 sample from mixture to a clean 250ml conical flask using a 10ml measuring cylinder and teat pipette
1c – Quench the reaction by adding 20cm3 of cold water
1d – Titrate sample with 1moldm-3 HCl in burette to determine the concentration of
NaOH
Stage 2: Use of results
2a – Plot the concentration of NaOH /moldm-3
(y axis) vs time/secs (x axis)
2b – Draw tangents t=30 secs & t=60 secs and calculate the gradients
2c – Rate = gradient which will allow you to determine the rate at two different
concentrations
Stage 3: Analysis
3a – Compare the change in concentration to change in rate.
3b – If the conc is x ½ and the rate x ½ = 1st order wrt NaOH as the concentration is
directly proportional to rate.
3c – If the conc is x ½ and the rate decreases by x ¼ = 2nd order wrt NaOH as
concentration2
is directly proportional to rate.
3d – If conc is x ½ and rate does not change, 0 order wrt to NaOH.

Question 17

Measuring the EMF of an electrochemical cell
t is difficult to ensure consistency with the setup of a standard hydrogen electrode. A Cu2+(aq) / Cu(s) electrode (Eo = +0.34 V) can be used as a secondary standard.
A student does an experiment to measure the standard electrode potential for the TiO2+(aq) / Ti(s) electrode using the Cu2+(aq) / Cu(s) electrode as a secondary standard.
A suitable solution containing the acidified TiO2+(aq) ion is formed when titanium(IV) oxysulfate (TiOSO4) is dissolved in 0.50 mol dm−3 sulfuric acid to make 50 cm3 of solution.
(b) Describe an experiment the student does to show that the standard electrode potential for the TiO2+(aq) / Ti(s) electrode is −0.88 V
The student is provided with:
• the Cu2+(aq) / Cu(s) electrode set up ready to use
• solid titanium(IV) oxysulfate (Mr = 159.9)
• 0.50 mol dm−3 sulfuric acid
• a strip of titanium
• laboratory apparatus and chemicals.
Your answer should include details of:
• how to prepare the solution of acidified TiO2+(aq)
• how to connect the electrodes
• measurements taken
• how the measurements should be used to calculate the standard
electrode potential for the TiO2+(aq) / Ti(s) electrode.

Answer 17

Stage 1: Preparing solution
(1a) Weigh 8.00 g TiOSO4
(1b) Dissolve in (0.50mol dm-3) sulfuric acid
(1c) transfer to volumetric flask and make up to the mark
Stage 2: Set up cell
(2a) piece of Ti immersed in (1 mol dm−3 acidified) TiO2+(aq) / the solution
(2b) (connect solutions with) salt bridge or description
(2c) (connect metals through high R) voltmeter
Stage 3: Measurements and calculation
(3a) record voltage/potential difference/emf of the cell
(3b) Ecell = ERHS – ELHS/Ecell = Ecopper – Etitanium
(3c) ELHS = ERHS – Ecell OR Ecell should be +1.22 V if Cu on RHS

Question 18

Separation of a species by thin-layer chromatography
A peptide is hydrolysed to form a solution containing a mixture of amino acids.
This mixture is then analysed by silica gel thin-layer chromatography (TLC) using a toxic solvent. The individual amino acids are identified from their Rf values.

Answer 18

Wearing plastic gloves to hold a TLC plate, draw a pencil line 1.5 cm
from the bottom of the plate.
2. Use a capillary tube to apply a very small drop of the solution of amino
acids to the mid-point of the pencil line.
3. Allow the spot to dry completely.
4. In the developing tank, add the developing solvent to a depth of not more
than 1 cm.
5. Place your TLC plate in the developing tank.
6. Allow the developing solvent to rise up the plate to the top.
7. Remove the plate and quickly mark the position of the solvent front with a
pencil.
8. Allow the plate to dry in a fume cupboard

Question 19

Why is it important to wear plastic gloves for Separation of a species by thin-layer chromatography

Answer 19

prevent contamination from the hands to the plate

Question 20

Why is it important to add developing solvent tp a depth of not more than 1cm3 in the Separation of a species by thin-layer chromatography

Answer 20

If solvent is too deep it will dussolve the mixture from the plate

Question 21

Why is it importat to allow the solvent to rise up the olate to the top for the Separation of a species by thin-layer chromatography

Answer 21

The Rf value can be calculated if the solvent front dows not reach the top of the plate

Question 22

Why is it important to allow the plate to dry in a fume cupboard in the Separation of a species by thin-layer chromatography

Answer 22

The solvent is toxic

Question 23

Outline the steps needed to locate the positions of the amino acids on the TLC plate and to determine their Rf values

Answer 23

Spray with developing agent or use UV
Measure distances from initial pencil line to the spots (x)
Measure distance from initial pencil line to solvent front line (y)
Rf value = x / y

Question 24

Acid & base titration

Answer 24

(1) Measure the pH of the acid solution and record.
(2) Add 1cm3 of the base solution
(3) Stir the mixture
(4) Measure the pH and record.
(5) Repeat the process until the base is in excess.
(6) Add base in smaller increments near the end point