We can now number of moles of calcium carbonate using this equation:
Number of moles = concentration x volume
1000
=0.1x25
1000
=0.0025 mol.
The following reaction took place:
H2SO4(aq) + Na2CO3 (aq) Na2SO4 (aq) +H2O(l) + CO2 (g)
There is a 1:1 ratio of sulphuric acid to sodium carbonate.
Therefore the number of moles of H2SO4 will be the same as those of Na2CO3.
Number of moles of H2SO4= 0.0025mols
Now we can work out the concentration of H2SO4 with this equation
Number of moles = concentration x volume
1000
0.0025 mol. = conc. x 25cm3
1000
Concentration (H2SO4) = 0.08 mol.dm-3 (2 d.p.)
Evaluating Evidence and procedures.
The final titration was an anomalous result – 34.30cm3. A result much larger than the average of 32.23cm3 (3.07cm3 larger). All the other results (apart from the first which is only a rough titration so that later ones can be more accurate) were 0.05 of the average.
The anomalous result could be down to a number of reasons. Such as the conical flask not being rinsed properly, leaving a small amount of water in the bottom, diluting the solution further. There may also have been errors in the readings. Or simply missing the end point by not swirling enough or turning off the burette quick enough.
Out of the 5 results collected 2 have been ignored, this leaves us with only 3 results to find an average with.
Another factor to consider is percentage error.
% Error = actual error/ size of measurement x 100
The balance that was used has an error of + or – 0.005g. 4 measurements were taken:
12.57g
% Error = 0.005/12.57x100
=0.04%
15.24g
% Error = 0.005/15.24x100
=0.03%
12.59g
% Error = 0.005/12.59x100
=0.04%
2.65g
% Error = 0.005/2.65x100
=0.19
One drop from the burette has a volume of approx. 0.05cm3, this gives an error of + or – 0.05. The average titre was 32.23cm3; therefore the following % error occurred:
0.05/32.23x100= 0.16%
There may also be error when using the pipette, if used correctly the error is + or – 0.06cm3 therefore:
0.06/25x100=0.24%
The volumetric flask could also produce an error; if the bottom of the meniscus rests on the calibration line the error is 0.2cm3. So:
0.2/250X100= 0.08%
The largest % errors are produced when using the burette and pipette. Therefore more care is to be taken when taking readings from them.
The total % error =0.78%. Therefore the results can be 0.78% larger or smaller than the results I collected: 0.78% of 32.23cm3= + or – 0.25. Therefore the exact result is between 31.98and 32.48.
The errors in procedure could arise in the following:
- Solution in volumetric flask partially mixed.
- Burette and pipette not rinsed properly
- Conical flask not rinsed properly between titrations.
- Acid not added drop by drop as the end point approached.
- Swirling not a continuous action, allowing the end point to be missed.
- Too much or too little indicator.
The middle 3 results are the only results accurate enough to use as evidence. If I were to use these results as evidence I would only include the middle three results and then continue until I had 10 results within 0.1 of each other. The % error of the experiment was very small (0.78%) this means the three useable results appear to be reasonably accurate.
There are many ways to improve the experiment:
- More repetitions, the more repetitions you do the more accurate your end point should be and the average would be more accurate as you are taking into account more results.
- Ensure the pipette, burette and volumetric flask readings are done at eye level on a horizontal surface. This makes the readings the readings more accurate.
- Use a thinner burette so that changes in volume are even more noticeable and lowering % error.
- Add the solution in drips rather than a steady stream. This would make the end point much more accurate.
If the experiment is performed in pairs it would make the burette readings more accurate, as 1 person could swirl the solution and the other could handle the burette. This would allow the swirling to be constant as the burette handler could concentrate on finding the end point accurately.
