As the stoichiometry of the reaction is one to one, therefore the number of moles of sodium carbonate is equal to the number of moles of sulphuric acid.
Previously in my coursework I worked out that I had 0.025 moles of sodium carbonate in my 250cm3 solution.
Number of moles of sodium carbonate in 25 cm3 = 0.025 = 0.0025 moles
10
Therefore the number of moles of sulphuric acid that reacted with the sodium carbonate = 0.0025 moles
Concentration of Sulphuric acid = 0.0025 = 0.098039215 mol/dm3
0.0255
= 0.098 mol/dm3 (2d.p.)
The aim of my experiment was to determine the unknown concentration of a sample of sulphuric (VI) acid, which is thought to have a concentration between 0.05 and 0.15 mol/dm3 my calculated concentration of 0.098 mol/dm3 is within this range.
Evaluation:
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In my experiment, all 3 of my accurate titration and my approximate titration had the same results, 25.50 cm3. These results tell me that my 250cm3 sodium carbonate solution was equally mixed in my volumetric flask before I pipetted 25cm3 of the solution into the conical flask.
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Whilst doing the titration, special care was taken when measuring the sodium carbonate in the pipette. The meniscus had to sit on the marking line to make up 25 cm3; this was also the same for reading the burette. The burette had to be read from where the bottom of the meniscus was, at eye level. As my results were identical for all 4 of my titrations, I know that I must have measured the same amount of solution each time. However, this doesn’t ensure that I used exactly 25cm3. This also applies for the volumetric flask. When making up the initial 250cm3 sodium carbonate solution, you cannot exceed the 250cm3 line; otherwise you cannot determine the correct number of moles in you solution. There may have been some errors in making up the solution to the line as it is difficult to determine whether the meniscus is on the line. This may mean that the concentration of the sodium carbonate solution is more dilute or more concentrated than what I calculated
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The stand for my burette doesn’t ensure the burette is completely straight. This could affect the readings for my solution if the meniscus isn’t straight.
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- The apparatus I use limits the accuracy of my experiment. Percentage uncertainty calculations enable me to evaluate how accurate/inaccurate my apparatus was.
Percentage Uncertainty calculations:
I can measure the percentage uncertainty of the following equipment I used:
- Digital scale / Balance
- Volumetric flask
- Pipette
- Burette
1. Percentage uncertainty of scale – measuring amount of sodium carbonate solid
0.005 x 100 = 0.189%
2.65
2. Percentage uncertainty of the volumetric flask
0.2 x 100 = 0.10%
250
3. Percentage uncertainty of the pipette
0.06 x 100 = 0.24%
25.0
4. Percentage error of the burette reading
0.05 x 100 = 0.196%
25.5
= 0.20%
Total percentage Uncertainty = 0.189 + 0.1 + 0.24 + 0.20 = 0.729%
I worked out the concentration of sulphuric acid to be 0.098mol/dm-3 and so 0.729% of 0.099 is
0.729% of 0.099 = 0.098 x 0.729 = 0.00071442
100
Therefore, the precision error is ± 0.00071442
Due to limitations of the equipment, the concentration of the acid is:
0.098 ± 0.00071442 mol/dm3
The concentration was 0.1mol/dm3. My percentage accuracy is calculated by
(My result – correct answer) = 0.098 – 0.1= -0.02 = 0.02%
Correct answer 0.1
To ensure accurate results you must:
- Add the same number of drops (3) of methyl orange indicator to the sodium carbonate solution. This ensures that you identify the same colour change each time, so you will record the reading as soon as the solution is neutralised.
- Read the meniscus from eye level, which ensures that I have recorded the correct reading from the burette.
- Wash the glassware with the appropriate solution to ensure the remains of the previous experiment didn’t affect the results.
Improvements to my experiment:
- A 4 point mass balance could be used, so the percentage uncertainty would be reduced even more, leading to more accurate results
- A graduated pipette could be used instead of a bulb pipette so that the percentage error is also reduced.
- Taking readings from a burette can be difficult as it is clear. To reduce the risk of misreading the volume, I would place a white piece of paper behind the burette, making it easier to see the meniscus.
- As the stoichiometry of the reaction in my experiment was one to one, it is vital that all the sodium carbonate dissolved in the solution, allowing me to correctly calculate concentration and the number of moles. Therefore, I would heat the sodium carbonate solution using a Bunsen burner to ensure the entire solid is dissolved. I would then allow the solution to cool before transferring it to the volumetric flask.
- In my experiment, I need to record the exact volume of sulphuric acid needed to neutralise the sodium carbonate solution. Therefore, noticing when my solution is neutralised is vital. To determine the colour change to a greater accuracy, I would use a colorimeter, which allows the absorbance of a solution at a particular frequency of visual light to be determined. This piece of apparatus would help me to know when the end point of experiment has been reached, when neutralisation occurs.
