Now I need to work out the concentration of hydrochloric acid.
Concentration = no. moles x 1000 / volume = 6.5 x 10 x 1000 / 25 = 0.026 mol dm
I now know that I want 0.026-mol dm of hydrochloric acid for the reaction to work. But I have been supplied with 2.00-mol solution. I want 250cm of dilute hydrochloric acid at 0.026-mol dm. The total no. moles in 250cm is unknown. 250 cm of the solution should be enough to repeat the experiment. To work out the total no.mole I do this:
No. moles = concentration x volume / 1000
This equation is rearranged to make:
Volume = no. moles x 1000 / concentration
So ,
1000cm contains 0.026 moles
500cm contains 0.013 moles
250cm contains 0.0065 moles
So, the volume of hydrochloric acid I need is:
Volume = no. moles x 1000 / concentration = 0.0065 x 1000 / 2 =3.25cm
Here is how I am going to carry out my experiment:
- Set up the apparatus and equipment
- Add the 3.25cm of hydrochloric acid to the volumetric flask using the graduated pipette to get the exact amount of hydrochloric acid, as this is the most accurate way of getting the right amount.
- Fill the same volumetric flask with distilled water until is reaches 250ml. This should now make my dilute hydrochloric acid with 0.0065 -mol dm. Now the hydrochloric acid is ready,
- Wash everything with tap water, then distilled water and finally with hydrochloric acid.
- Fill the burette with 50cm of the dilute hydrochloric acid making sure the bottom of the meniscus is reading at 50 on the burette. Using a funnel to put the dilute hydrochloric acid in the burette increases safety against spillage.
- Fill the conical flask with 25cm of limewater using the squeezing pump and pipette.
- Add three drops of methyl orange to the limewater and mix until it turns a light yellow depending on the concentration of the methyl orange.
- Goggles must be worn when handling hydrochloric acid as its hazardous to your skin.
- Place the conical flask containing limewater onto the white tile.
- Start to release the dilute hydrochloric acid steadily, starting of constantly then proceeding into small squirts and then drips whilst mixing. This will make sure you will use the exact amount of hydrochloric acid when the colour changes. This improves accuracy and reliability.
- Repeat all the tests at least three times (Time dependent) so that an average can be obtained. Repeating the experiments several times will help to produce better and more accurate results, as any inaccuracies in one experiment should be compensated for by the other experiments. Note all the results in a table.
One thing I have to decide is what indicator I want to use. The indicators methyl orange and phenolphthalein are pretty much one-three drops change and a clear change. The pink/colourless change for phenolphthalein is very definite, the yellow/pink change for methyl orange is clear too. Because these are clear it means I can tell when the reaction has finished easier than using other indicators. Limewater, which is calcium hydroxide, is a strong alkali, even though it is only sparing soluble. Its pH is 14. Hydrochloric acid is a strong acid. This means that even if you have quite a weak concentration it completely dissociates in water into H3O+ ions and OH- ions giving a pH of 1 (The pH is a measure of how many hydrogen ions are present in solution).
For the reaction of Calcium hydroxide and Hydrochloric acid I would use methyl orange. It changes colour at around pH 4.4 and will change from yellow in acid to pink in alkali by the addition of limewater. To make this experiment as accurate as possible, the pipette, burette and volumetric flask only have a percentage error of + or – 0.1%. This is only a small error. Also making sure the hydrochloric acid is poured into the burette carefully using a funnel will prevent the hydrochloric acid pouring down the side of the burette.
Results
I worked out the average titre like this:
Mean = 13.2 + 13.3 + 13.2 / 3 = 13.2
Calculating the concentration of limewater
Ca(OH) 2 (aq) + HCl (aq) --------> CaCl2 (aq) + 2H2O (l)
Concentration of limewater = concentration of HCl x volume of HCl / volume of
Limewater
Volume of hydrochloric acid = 13.2cm
0.0065 x 13.2 / 1000 = 0.0000858 moles
Ca(OH) 2 (aq) + HCl (aq) --------> CaCl2 (aq) + 2H2O (l)
1 mole of Ca(OH) 2 : 2 moles of HCl
0.0000429 : 0.0000858
There is 0.0000429 moles of limewater in 25cm
1dm contains = 0.0000429 x 40 = 0.001716
For gdm = 0.001716 x Mr of Ca(OH) 2 = 0.001716 x 74.1 = 0.1271556 mol gdm
Mr of limewater = 40.1 + (16 x 2) + (1 x 2) = 74.1
Conclusion
There were no anomalous results as all my titres where between 0.1cm of each other. This implies that the method was reliable as when it was repeated, similar results were found. This does not necessarily mean that the results obtained are accurate, but I would expect my results to be reliable within a certain percentage error region.
Through research, I found that both indicators had different pH. This was shown by the results from methyl orange. The pH for methyl orange is around pH 3.2 – 4.4 suggesting it is an acid. The reason for the more hydrochloric used to change the colour of the methyl orange is because there are two acids reacting with each other. Also methyl orange has two moles of hydrochloric acid to titrate instead of one mole with the phenolphthalein suggesting that the phenolphthalein would be harder to spot a colour change as only a small amount of hydrochloric acid is needed.
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. 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.
As you can see the concentration of the limewater was higher than the concentration of dilute hydrochloric acid suggest the change was around 13.2cm. 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 Hydrochloric acid, which is why I have used a concentration of 0.0065-mol dm rather than 0.01-mol dm.
Another error is that contamination is easily done as quite often you can forget which side of the burette you put the hydrochloric acid and which side you put the limewater.
My results are reliable as they are consistent with each other and I have managed to refrain from making any large errors.
Improvements
There are many things I did or should have done when carrying out a titration to ensure that the results are as accurate and reliable as possible:
1. Always wash out the apparatus with distilled water before you start.
2. When you have filled the burette make sure that you have removed the air bubble from below the burette’s tap.
3. When transferring the solution into the conical flask always use a graduated pipette. Ensure that the bottom of the meniscus is on the line.
4. Always leave the last drop in the graduated pipette – it is calibrated this way.
5. When adding indicator add only enough drops to get a reasonable colour. Too many drops will affect the end point and hence the accuracy of your results.
6. Always use a white tile so that the colour change at the end point is clear.
7. Before carrying out your first titration remove the filter funnel you used to fill the burette.
8. Record the starting volume and the final volume when the end point is reached. Don’t forget to record the volumes to 2 decimal places – i.e. 0.00 means on the line 0.05 means between the lines.
9. Repeat the titrations until you get concordant results i.e. 2 identical titres or 3 within 0.1cm3
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.
