2022 MGS 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 pressure, chemical reacts with moisture or oxygen in the air. Sources of Error and its effects on the experiment 1. Hydrogen peroxide decomposes into water and oxygen at room temperature and pressure and this will result in loss of the chemical while 2. aqueous potassium iodide can be oxidised to iodine on exposure to oxygen and this will result in the loss of the chemical. 2. Sodium hydroxide and some salts like zinc chloride and calcium chloride can absorb moisture in the air and dissolve in it. This will increase the mass of the solid. In titration experiment, if methyl orange is used as an indicator, it will change colour between pH 3 – 4. However if the pH of the resulting salt solution at point of neutralisation is 7 then the volume of the titre will be smaller than what it should have been since the methyl orange changes colour before the acid or alkali is completely neutralised to form the salt solution at pH 7.

5 Example 1 (a) State two sources of error in the experiment. Same size of marble was used does not mean same mass of marble was used in each experiment. This will lead to inaccurate time measurement. Starting the timer is a source of error because one stopwatch is used for all the experiment. Or the experiment for each solution is done only once. (b) Suggest two improvements to reduce the sources of errors in the experiments. Weigh a fixed mass of marble chip for each solution. Time individual experiment. Repeat the experiment at least 2 times for each solution to get the average time taken.

6 Example 2 – Excerpt from Experiment 4.12 (iii) Suggest an improvement for the experiment. Store solution Q in a dark bottle/ dark room before use or Prepare solution Q just before the experiment. Or Conduct the experiment in a cool place. Unacceptable suggestions: Conduct the experiment in a dark room/ r oom without sunlight/ room with all curtains drawn / Use a dark conical flask or cover the conical flask

7 3 Purpose of Experiment The aim of an experiment determines the variable to be measured or dependent variable in the investigation of the effect of the independent variable. When the dependent variables are either temperature change or time taken for observation to be made, sources of error can be caused by inherent inaccuracies when these dependent variables are measured. Example 1 – Excerpt from Experiment 3.3 (a) Experiment 1 Use a measuring cylinder to measure 50 cm 3 of aqueous sodium hydroxide and pour into the polystyrene cup provided. Put the cup into a 250 cm3 beaker. Measure the initial temperature (To) of the solution. Leave the thermometer in the Styrofoam cup. Record To in the table on the next page. Fill the burette with the solution of acid A provided. Add 5.00 cm3 of acid A to the aqueous sodium hydroxide in the cup and stir the mixture. Measure and record the maximum temperature (T i) of the solution in the table below. Add a further 5.00 cm 3 of acid A to the cup and stir the mixture. Measure and record the maximum temperature (Ti) of the mixture in the table below. Continue to add 5.00 cm 3 portions of acid A to the cup, until a total volume of 40.00 cm 3 of acid has been added. Stir after each addition and measure and record the temperatures (Ti) in the table. Calculate the values of temperature rise (T i – To). Tabulate all your results to show the total volumes of hydrochloric acid added and the corresponding values of highest temperature reached (Ti) and temperature rise (Ti – To). At the end of this experiment, pour the solution away and rinse the Styrofoam cup. (a) State one source of error in the experiment and how it will affect the results of the experiment. (results is about temperature) Heat lost to the surrounding due to poor insulation of the mixture. This will cause the temperatures measured to be lower than expected. (NOT: actual) Or slow run of the acid from burette causes more heat lost to the surroundings. This caused temperatures measured/recorded to be lower than expected. (b) Suggest an improvement to reduce the source of error in the experiments. Cover the polystyrene cup to minimise heat lost to the surrounding. Or Use measuring cylinder instead of burette Heat experiment

8 Example 2 – Excerpt from Experiment 4.4 Instructions You are required to carry out five experiments using the apparatus shown. (a) Experiment 1 Use a measuring cylinder to measure 50 cm3 of aqueous sodium thiosulfate into a conical flask. Place the conical flask on the printed X provided. Fill the small measuring cylinder with 10 cm3 of the hydrochloric acid provided. Add acid to