2022_Paper_3_Thinking_Behind_Doing_T

2 Name : _______________________( ) Sec 4 ________ Sources of Errors in Chemistry Experiments Sources of errors in an experiment are errors that are inherent in an experiment due to the design of the experiment. It is not due to the mistakes made by the experimenter. These sources of errors can be caused by the following aspects in the design of an experiment: 1. apparatus, 2. materials, 3. purpose of experiment. 1 Apparatus The criteria for selecting an apparatus for an experiment are as follows: a) function b) degree of accuracy c) ease of use Usually if ease of use is chosen as a criteria, the degree of accuracy may be compromised and it can be a source of error for the experiment. An obvious improvement to this experiment is to use an apparatus of an appropriate degree of accuracy. Example 1

3 (a) State two sources of error in the experiment. 1. The measuring cylinder used to measure volume of solution F is not as precise as a pipette. It can only measure accurately to 0.5 cm3 of solution. 2. The experiment was conducted only once to find volumes of HCl used. OR Adding 1.00 cm3 of HCl solution each time means that the end point cannot be determined accurately. Note: pipette measure fixed volume e.g. 25.0 cm3, 20.0 cm3 … (b) Suggest two improvements to reduce the sources of errors in the experiments. 1. Use a pipette to measure 25 cm3 of F 2. Repeat the experiment 3 times for each solution F and G to obtain average volume of HCl used in the titration 3.Add HCl dropwise near the end-point to obtain accurate volume of HCl used. Example 2 (a) Suggest the purpose of the starch solution. To detect the presence of iodine. Iodine reacts with starch to form a complex (blue-black colour) (b) Suggest one advantage of using a pipette to measure the volume of hydrogen peroxide. Pipette is a more precise apparatus and hence the volume of hydrogen peroxide measured is more accurate. (conical flask) (using a teat pipette)

4 (c) Suggest and explain one disadvantage of using a pipette to measure the volume of the hydrogen peroxide. Solution is slow to run out of the pipette/difficult to know when to start timer. This leads to inaccurate time measurements. (d) Explain one disadvantage of using a beaker instead of a conical flask. It is more difficult to swirl or mix the chemicals without spillage. 2 Materials The materials used in an experiment refer to the chemicals , reagents, indicators or wooden splints required. Sources of error can be due to: 1. Using different sizes of marble chips for each experiment, timing the experiment all at the same time 2. Stability of chemicals e.g. hydrogen peroxide decomposes at room temperature and pres