- Add a few drops of methyl orange indicator.
- Wash the burette with some of the acidic solution being used. Maker sure the solution runs into the tip of the burette.
- Once washed fill the burette up to the meniscus point at 0 point, this should be read at eye level.
- Arrange the Apparatus as below.
In order for the experiment to be accurate and reliable it will be necessary to dilute the Hydrochloric acid significantly. It is in my belief that a concentration of 0.1mol dm?3 would enable me to achieve reliable results. This would require a dilution factor of twenty. I will be making 500 cm3 of dilute hydrochloric acid. I will also be using 25cm3 of Limewater.
In order to reduce errors it is necessary to choose accurate and reliable equipment. That is why I have chosen the pipette, volumetric flask and burette. All of these instruments have an accuracy of ±0.05cm3, which is suitable for my experiment and should produce accurate and reliable results. In making my choice of indicator I also had to be very careful. The Limewater used will be a relatively weak base so it will be appropriate to use methyl orange as it has an end point on the ph scale between 8-5. Whereas phenolphthalein indicator has an end point much higher up in the ph scale. So if I were to use the phenolphthalein indicator then the end point (colour change) would be reached before the equivalence point (when the chemicals, acid-base break up).
Implementing
Safety is very important and it is necessary to follow safety procedures. It is important to wear eye protection when working with any acid. Also long hair should be tied back. Also as the chemicals used are corrosive it is important to take extra care while working around them.
In order to start with the experiment it is required of me to dilute the HCl. An accurate way of doing this is to use a volumetric flask and pipette. I want to dilute the acid to 0.1 moles, so I pipette 25.0cm3 of HCl from the original bottle and place it in the 500cm3 volumetric flask. Then I add 475cm3 of distilled water to the same volumetric flask, I measure the amount of water by looking at the graduated mark on the flask and fill until the 500cm3 mark.
Once I have obtained the correct concentration of Hydrochloric acid I can precede to the rest of the experiment. First I must set up the apparatus as I have indicated in my diagram. I will be using 25.0cm3 of limewater and I will transfer it into the conical flask using a 25.0cm3 pipette. It is also important to add the indicator into the conical flask so that the neutralisation point can be obtained, where five drops of methyl orange should be appropriate. When the indicator is added the colour of the solution is light, clear, orange and this should turn to pink by the end of the titration. In order to obtain consistent and reliable results it is necessary to repeat the experiment several times. After this it the results must be noted in a table as I have indicated in the table below.
Titration 1 Titration 2 Titration 3
Final Reading 6.70 cm3 12.30 cm3 17.90 cm3
Initial Reading 1.00 cm3 6.70 cm3 12.30 cm3
Titre 5.70 cm3 5.60 cm3 5.60 cm3
Average titre
5.70cm3 + 5.60cm3 + 5.60cm3 = 5.63cm3
3
Conclusion
Number of moles of HCl:
1dm3= 10*10*10cm3= 1000cm3
Number of moles= concentration * volume
0.10 mol dm? 3* 5.63cm3 = 5.63*10?@ mol
1000
Number of moles of Ca(OH)2 (Calcium hydroxide):
Reacting ratio: 1 Ca(OH)2 : 2 Hcl
5.63*10¯@ mol = 2.82*10?@mol
2
Concentration of Limewater in mol dm?3:
25.0cm3 = 25.0 dm3 = 2.50*10?2 dm3
1000
2.82*10?@ mol dm?3
2.50*10?2
= 1.13*10?2 mol dm?3
Concentration of Limewater in g dm?3:
Formula mass (Ca(OH)2)= 40.1+(16+1)*2 = 74.1g
Concentration of Ca(OH)2= 74.1* 1.13*10?2 g dm?3
= 0.83733 g dm?3
= 0.84 g dm?3?
I conclude that my results are quite accurate as they are near to the 1g dm?3 estimate given to us. I am satisfied with my results but will need to go through any source of error. Also the fact that my results are consistent with each other proves that they are reliable.
Evaluation
I feel that my practical went well and was suitable for the task given. This is proven by the fact that I achieved consistency and proves that this experiment can be both reliable and accurate. In the titration out of three, I got 2 identical titres and one that was only 0.1cm3 off. Also by choosing relatively accurate instruments I minimise error through instrument limitations. The pipette, volumetric flask and burette have an accuracy of approximately ± 0.05cm3, this is far more accurate when compared to that of the burette which has an accuracy of ±5.00cm3. So any error through equipment is minimal, yet it still has to be taken into consideration.
I feel that there was little limitation in the procedure and any limitation was minimal. An example of this is the minute splashes that occur in the conical flask during the titration, which some of it gets on the walls of the conical flask. The pipette that I was using was holding back some of the solution each time but I dispersed of this by blocking of the top end and wrapping my hand around the pipette, warming the interior slightly and pushing the drop out.
Some errors are likely to come from measurements. A source of error would be parallax error, which would involve the meniscus. The parallax error would be present in the burette and pipette. This error must also be taken into consideration. Another form of human error would be the judgement in colour change. In titration it is impossible for one to judge the solution during the end point of the indicator to be the exact same colour all three times with just the naked eye. This is another limitation in the procedure. During titration the colour change can be rapid to counter this I have used a weak concentration of HCl, which is why I have used a concentration of 0.01 mol dm?3 rather than 0.02 mol dm?3.
The experiment can be improved by using a computer and a real time video camera. If the computer could hold the colour of the initial titration then one could do the second titration while looking at the computer screen, so both titrations would be stopped at exactly the same colour. This would totally reduce the human error when it comes to the colour change of the indicator, making the results more fair, accurate and reliable. Another way I could have improved the experiment was if I was to use a larger quantity of limewater for the titrations and, as a whole would reduce the percentage error. Also by repeating the experiment several more times I could have obtained a better average result.
My results are reliable as they are consistent with each other and I have managed to refrain from making any large errors.
