CIE A-level Chemistry Paper 3 Tips

Chemistry practical is the hardest practical exam out of the three sciences. Two hours are really too short to finish the paper. So, time management is really significant for this paper. 


Qualitative Analysis

1. Titration

Titration is a very common questions that will be asked. Before you start conducting the experiment, read the questions carefully. Then, draw tables for you to record your readings.
Normally they will want you to record a rough titration and two accurate titration. For the table, you need three rows, which is :

-Final burette reading/ cm3
-Initial burette reading/ cm3    
-Volume of __ added /cm3

Always remember the units! Always put initial burette reading as 0.00. Value of the two accurate titration must be at most 0.10 difference only!


The next question will most likely be, find the suitale value of ___.
Since you have two readings for ___. Just calculate the mean to get the answer.


They will also ask you to calculate the mole, concentration or volume of the solution. So, what if they give you g dm-3 instead of mol dm-3? How are you going to calculate the mole?

-If they give you g dm-3 and also g mol-1 which is the relative molecular mass, you can find the concentration, mol dm-3 by dividing. Once you get mol dm-3, you can calculate the mole.

Always, always use ratio to calculate. For example, they will ask you to find the number of mole of FA1 in 250cm3. If FA1 has 0.01 mole in 25 cm3,  FA1 then has 0.1 mole in 250cm3.

250/25 x 0.01 = 0.1 mol

Maximum error in volume run from the burette, 
0.05 x 2 = 0.10 cm3

Maximum percentage error for burette,
0.10/(Volume of solution used) x 100 %

Maximum error of mass, 
0.01/2 = 0.005

Maximum percentage error of mass,
(2 x 0.005) / (mass of ___) x 100%


**Always, always, always remember to put your answer in 3 significant figure!

2. Graph

-Draw the x-axis and y-axis and label it. Give units for each of the label. 
-Use suitable scale for your graph, like factors of 2, 5 and 10.
-The graph can start from other numbers instead of 0.
-The plot must be smaller than the small box. 
-Graph must be larger than half of the graph paper.
-Normally there will be 2 lines; they can be straight line or curve line, depends on the question.

3. Experiment with heating

If the question ask you to conduct the experiment with 2 different masses of  ___, you will need three column for your table because you need to record two different readings.

-mass of tube + ___ /g
-mass of empty tube /g
-mass of tube + residual /g 
-mass of residual /g
-mass of water lost /g


If the questions only want you to conduct the experiment with only 1 mass, you only need two column as you only have one type of readings to be recorded. However, you need to draw three row next to 'mass of tube + residual /g' to record the readings three times. This is because you need to heat the tube for at least 3 times to get the residual with the maximum water lost.  

The last reading of mass of tube + residual with be used in your calculation. 

the formula Q = mc T is important. 






Qualitative analysis

-Use the qualitative analysis notes wisely! If your answers are anything related to the reactions of aqueous cations, reactions of anions and tests for gases, make sure to copy every words from the qualitative analysis notes to the answer spaces.

-When your answer has gas in it, make sure you write out the name of the gas too. For example, DO NOT write gas turns damp red litmus paper blue, but write NH3 gas turns damp red litmus paper blue. 

-For any colour change, make sure you indicate the initial and final colour. Do not just write the final colour change.

-If precipitate is seen, note down the colour and solubility (refer to qualitative analysis notes!)

-If gas bubbled are formed, it means there is gas released. The gas test can be found on the qualitative analysis notes. 

-Write down if you see any efferverscence occurs. 

Frequently asked questions...

- (heating) Why total volume of solution used was kept constant in each experiment?
A: change in volume give difference change in temperature for same energy produced.

-(heating) How to ensure all water of crystallisation had been removed?
A: repeat the heating, cooling and weighing process for a few time until a constant mass is obtained.

-(involve metal) Stat the observation which confirm sulfuric acid is in excess.
A: magnesium ribbon dissolves completely in sulfuric acid.

-(involve metal) Which temperature change has hgiher percentage error?
A: test with shorter magnesiu ribbon.

-(If asked to compare two reactions) Reaction 1 is more accurate. Why is reaction 2 less accurate?
A: lower enthalpy change and higher percentage error.

-What is the effect on percentage error in mass if greater mass is used?
A: percentage error in mass increases.

-How repeating experiment would lead to greater reliability?
A: to show consistent result / to eliminate anomalous result

Recently questions with this type of experiment is very popular. You can find it in May/June 2016 paper as well as the recent one, Oct/Nov 2016 paper.

What should you record for your table then?

Mass of container + (chips used) /g
Mass of container /g
Mass of (chips used) /g 
Final volume of gas collected /cm3

The gas collected is CO2.


They also like to ask about inaccuracy and want you to explain a modification to help reduce the difference in volume. 

I: Some CO2 evolved dissolved in the water (less volume than expected)
M: Use other substances like oil which CO2 is not soluble.

I: CO2 lost before the bung replace
M: arrange the marble chips in flask so mixing is carried out after bung replaced/ use larger lump of solid

I: delay in starting stopwatch (greater volume than expected)
M: ask for assistance