I will be using a volumetric flask which can hold 250cm3 of liquid. Therefore, in order to make a 1.0 M solution of citric acid I will require 210 / 4 as I am not using a whole litre of water.
210 / 4 = 52.5g of citric acid.
However, I need to make a 0.33 M solution and therefore I will require 52.5 / 3 grams of citric acid.
52.5 / 3 = 17.5g of citric acid required to make a 0.33 M solution of citric acid, assuming that the acid is 100% pure. It is clear that the concentration of citric acid in my solution will not be 0.33 M, but this calculation allows me to make an approximation and therefore perform my titration experiment.
Method
1) I will carefully weigh out 17.5g of citric acid and dissolve it in distilled water. I will then take the dissolved citric acid and place it carefully in a volumetric flask of volume 250cm3. I will then wash out the beaker, in which I have dissolved the citric acid carefully, with distilled water and add the washings to the volumetric flask. I will repeat this process three times to ensure that all of the citric acid has been transferred from the beaker to the volumetric flask. I will then fill up the volumetric flask with distilled water adding the last few drops with pipette filler until I have exactly 250cm3 of water in the volumetric flask. I will then mix the concentrated citric acid with the water by turning the flask upside down and then upright three times.
2) I will then use the citric acid solution that I have made up, to fill the burette up to the 0cm3 mark. I will fill it by first pouring the citric acid solution into a small beaker and then pouring this carefully into a small filter funnel, which is inserted in the burette.
3) I will then take a pipette and measure out exactly 25cm3 of the 1.0 M sodium hydroxide solution and place this carefully into a conical flask along with several drops of phenolphthalein indicator.
4) I will then do a trial titration to gauge the approximate amounts necessary for neutralisation to occur. I will be able to see this when the phenolphthalein indictor, which is pink in alkali, turns colourless.
5) I will then do 4 accurate titration experiments in order to ensure that my results are correct. I will then record the average value for my results.
6) I should now be able to calculate the percentage purity of the citric acid using my average value for the amount of citric acid required to neutralise the 25cm3 of sodium hydroxide.
Fair Test
In order to keep my experiment as accurate as possible, I will keep the amount of sodium hydroxide in the conical flask constant at 25cm3, I will also keep the concentration of the sodium hydroxide constant at 1.0 M. I will measure the amount of citric acid needed to turn the phenolphthalein colourless, and record this accurately.
Safety
Throughout this experiment, I will ensure that I wear safety goggles and a protective lab coat so that I my eyes and clothes are protected from the acids and alkalis and no damage is likely to be caused to either.
Results
Calculation to work out the percentage purity of the citric acid
Volume sodium hydroxide used: Volume citric acid
25cm3 : 45.3 cm3
Number of grams in 25 cm3 of sodium hydroxide = 1.0M contains 1 mole of sodium hydroxide per 1 dm3 . Therefore in 25 cm3 (1 dm3 / 40 = 25 cm3) amount of moles of sodium hydroxide in 25 cm3 = 1.0/40 = 0.025 moles.
Number of moles = Mass/RMM
O.25 = Mass/40 (RMM NaOH = 23+16+1 = 40)
0.25*40 = Mass
1g = Mass
Therefore, the mass of sodium hydroxide in 25 cm3 = 1g
In 250 cm3 of impure citric acid solution there is 17.5g of impure citric acid.
Therefore in 1 cm3 there is 17.5/250 grams of impure citric acid.
17.5/250=0.07
I used 45.3 cm3 of impure citric acid solution.
0.07*25 = 3.171g
Ratios of volume should be 1:1 as I calculated the exact amount of pure citric acid to exactly neutralise the 25 cm3 of sodium hydroxide solution in a 1:1 ratio. Therefore if the citric acid were pure, it would take 25 cm3 to neutralise the sodium hydroxide.
Therefore the correct volume of citric acid is:
0.07*25 = 1.75g
Percentage purity = (correct value/impure value) * 100
Percentage purity of the citric acid = (1.75/3.171) * 100
Conclusion
According to my results, I can conclude that the percentage purity of the sample of citric acid I have been given is 55%
Evaluation
I think that my results are accurate within the boundaries of experimental error. However, I think that at this early stage of the IB course my experimental technique in areas such as doing titrations and making up standard solutions leaves a good deal to be desired and I could certainly make my results more accurate if I took into account the following factors in future.
1) I find that when I transfer the concentrated solution into the volumetric solution whilst making the standard solution, I have often dissolved the citric acid in too much water and therefore, I have difficulty in washing out the three times as this would overfill the flask. I think to improve my technique in this area I need to be more patient whilst dissolving the solid in the water, rather than assuming that my solution is saturated too early.
2) I also find that in transferring the concentrated solution and the washings into the volumetric flask, a drop of liquid will often trickle down the side of the beaker and is consequently lost. This makes the experiment considerably inaccurate. I think that this situation is due to the size of the funnel that I am using, as this means that I have to pour the liquid very slowly and consequently drips escape. To rectify this predicament, I could either use a larger funnel, or if this would not fit in the neck of the volumetric flask, I could control the flow of the liquid by using a large funnel above a small one. I think that the solution is particularly good as it would allow me to keep the flow of liquid constant, whilst any escaping drips would fall into the larger funnel.
3) It is also possible that my lack of experience at reading values from burettes, volumetric flasks and pipettes may give rise to minor inaccuracies. However, I would consider these inaccuracies to be extremely minor as I am fairly confident in my own ability to read off accurately. I would say, however, that my own lack of experience in using a burette and finding the exact point when the indicator changes colour may have given rise to a degree of inaccuracy. It is quite possible that I have misread the colour change on one or more occasions, or even added too many drops to allow the indicator to change colour.
However, despite these minor points which could possibly give rise to some degree of inaccuracy, I am confident enough in my experimental technique to say that my results are fairly accurate within the limits of experimental error.
