Variables
The control variables, which are the factors I shall be keeping the same, are that I shall use the same solution of sodium carbonate for each titration, use the same batch of sulphuric acid because there’s no guarantee the concentration of it is always the same for each batch, and the temperature will be constant.
Procedure
- Collect all the apparatus needed, carefully placing them into the centre of the table to ensure nothing gets pushed off the table or broken.
- Wash all your glass-ware before conducting the experiment to clean out any impurities left behind from previous experiments. This ensures the solution is not disturbed by any unwanted particles.
- Weigh your watch glass on the weighing scales and record its mass.
- Place your watch glass on the scales and set them to 0. Put the sample of sodium carbonate on the watch glass and weigh it. Record the mass.
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Use a flat surface to put the sodium carbonate into the clean beaker. Add the distilled water to it drop at a time, and stir with the glass rod. Keep adding the distilled water until the sample dissolves. The aim is to use as little water as possible.
- Re-weigh your watch glass to ensure all the sodium carbonate is in the solution. It should be the same as the first time you weighed it showing all of the sodium carbonate has been used. This increases the accuracy of your experiment. If it is more than the original mass you need to wash the watch glass with distilled water and add that to the solution. This technique ensures that all the sodium carbonate is transferred into the beaker.
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Swirl the solution around in the beaker. Transfer the contents of the beaker into a clean 250cm³ volumetric flask using a funnel. Clean the beaker, glass rod and funnel with distilled water and add that to the volumetric flask.
- Label your solution so that it isn’t confused with another. This acts as your control and allows accuracy and reliability.
- Add distilled water to the volumetric flask until it is about 1cm below the graduation mark.
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Now add the distilled water using a clean dropping pipette until the bottom of the meniscus lies on the graduation mark.
- Stopper the flask and shake it a few times to mix the solution. It should come back to the same level. At this point show the solution to your teacher.
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Remove the stopper. Take the 25cm³ Pipette, holding it from the top, as it may heat and expand. Place the tip of the pipette ¾ of the way down of the beaker and withdraw 25cm³ of the solution using the pipette filler. Transfer this to a clean 250cm³ conical flask.
- Put 3 drops of methyl orange indicator to the conical flask.
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Take a new clean and dry beaker and fill it with sulphuric acid. Fill the burette with this using a funnel.
- Run a little of the sulphuric acid through the tap of the burette. This releases any trapped air from the burette. Make sure you fill the burette at or below eye level so that no acid can harm your face (preferably on a stool). Record the reading on the burette.
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As you are looking for a colour change, place a white tile underneath the conical flask.
- Add the sulphuric acid to the solution drop by drop as one drop can trigger a colour change and accuracy is needed. Swirl the flask after each addition otherwise the titration can be misleading. Towards the end of the experiment, the solution will begin to turn pale green. This is when you know the titration is coming to an end. The end point of the titration is when you swirl the solution and it stays colourless. Close the tap.
- Record the final burette reading. The difference between the first burette reading and the final burette reading is the volume of solution you have run out into the flask. This is your titre reading.
- Record your results and calculate the average titre:
Average titre = Total titre reading (cm³)
(cm³)
Number of titrations
- Repeat this experiment as many times as you need until you get 3 results within 0.1 of each other.
N.B All the apparatus should be thoroughly cleaned between each titration.
Risk Assessment
Safety glasses must be worn to protect the eyes from splash-back of any solution or sulphuric acid. In addition, a lab coat should be worn to protect clothes, hair should be tied back, broken skin should be covered with plasters and stools need to be safely tucked under desks.
All the apparatus will be placed in the centre of the desk so that nothing is in danger of falling off the desk and breaking. If any equipment does break, it should be placed into the broken glass bin with a dustpan and brush, and the teacher should be notified. Special caution should be taken when handling the pipette because it is fragile.
The titration needs to be carried out on a flat surface at all times, and should be done standing up not sitting, so that you are not at risk from the solution or acid falling on you. Fill the burette slowly with sulphuric acid at or below eye level to ensure safety from spillage and splash-back.
Results
Average Titre = 165.9 cm³
6
= 27.65 cm³
Average Titre of 3 closest figures = 27.10 cm³ + 27.15 cm³ + 27.20 cm³
3
= 27.15 cm³
This is the volume of sulphuric acid used in the titration.
Analysing and Concluding
Calculating the concentration of acid:
By calculating the concentration of sodium carbonate (Na2CO3), I can find out the concentration of sulphuric acid in the solution.
Concentration = Mass of Na2CO3 x 1000 cm³
of Na2CO3 RAM of Na2CO3 250 cm³
= 2.63g x 1000 cm³
- 250 cm³
= 0.1 mol dm-³
The concentration used for every 25 cm³ and for the solution as a whole is always the same.
Amount of = Concentration of x 25 cm³
Na2CO3 Na2CO3 1000 cm³
= 0.1 x 0.025
= 0.0025 mols
The amount of sodium carbonate and amount of sulphuric acid in the solution is equal to one another.
Amount of = 0.0025 mols
H2SO4
Concentration = Amount of H2SO4 ÷ Volume of H2SO4 (average titre)
Of H2SO4 1000
= 0.0025 ÷ 27.65 cm³
1000 cm³
= 0.09 mol dm-³
Evaluation
Generally, I feel the experiment was successful and was carried out with safety, fairness, and as much accuracy as possible. I think that the results I obtained were sufficient enough for me to come to decisive conclusions.
In preparation for the experiment, I carried out a rough titration. This gave me an approximation as to where about the titration was most likely to come to an end. This proved successful with the first two titrations, however there onwards the titres were smaller values than them and closer together. Therefore, I think the first two titrations are anomalous results, because they are relatively higher and further apart in value. These may have occurred because the titrations were carried out on different days on which I used different indicator solutions, so their concentrations may have differed slightly. Also, any unused sulphuric acid was always put back into the batch so although I used the same batch of sulphuric acid on both days there could have been contamination, affecting my results.
The equipment I used was calibrated making it more accurate and reliable, because each piece of apparatus was the same as another of its type. When making up the solution in the volumetric flask, withdrawing 25cm³ of it using the pipette, and reading from the burette, I ensured the bottom of the meniscus was on the graduation mark, reducing the likelihood of errors. I washed glassware before each titration to make sure there was no contamination from the previous practical, and so that it didn’t change the quantities and concentrations I used. Also, I read the burette to two decimal places, making it more accurate, and readings were always taken at eye level.
I have calculated the percentage errors involved in my measurements to indicate any inaccuracies. The errors are ± because they can be larger or smaller than the percentage error value.
The solution made up in the volumetric flask needed thorough shaking before each titration, to make the results fair and reliable. However, I only did this when I first made up the solution, so this may have caused my results to be higher than they should have been. My results may be inaccurate. This is because sometimes while I was swirling the solution in the conical flask and adding sulphuric acid to it drop by drop, I wasn’t quick enough to turn off the tap. So the solution passed its colourless state and turned slightly purple or blue. This may have caused my titres to be larger. Also, the pipette may not have always fully emptied.
Overall however, I think my results are relatively accurate and reliable, because I cleaned the equipment after each titration, I took measurements reading from the bottom of the meniscus, I used calibrated equipment, and the same batch of sulphuric acid and Na2CO3 solution every time.
To improve my procedure I think I could carry out two rough titrations instead of the one. In this way, it would allow me to expect a value between the two. I would also label a bottle of methyl orange indicator and use that each time so that the concentration of it cannot vary, ensuring the titration results are more accurate. When emptying the contents of the pipette into the conical flask, I would allow the pipette to fully drain until the last drop by touching the side of the flask with the pipette. This ensures the full 25 cm³ quantity is used each time. Before a titration I would ensure I shake the volumetric flask thoroughly so that the solution is of even concentration, making my experiment reliable and accurate.
