1/10 x 250 = 25cm3 hydrochloric acid
I will therefore need 225cm3 water to make up this concentration as well as ncm3 of water to wash out the other apparatus.
I predict that I will need a volume of about 25cm3 of calcium hydroxide for each titration, but can not say the exact total amount I require as I do not know how accurately I will be able to carry out the titration.
As well as this I require 3 drops of methyl orange indicator per titration and once again I can therefore not say the total amount I need as I don’t know how many titrations I will carry out.
Justification of Method
During this experiment I have decided to use a burette as it has a large resolution- i.e. it can be read very accurately. This will enable me to obtain more reliable results and therefore to calculate a more reliable reading for the concentration of the sulphuric acid.
For the same purpose I have used the volumetric flask as I can accurately make up a concentration of hydrochloric acid/ water solution. The instruments I am using are mainly all precision instruments which will allow me to make accurate readings.
The fact that I am rinsing things I have placed reactants in also make my results more accurate as I will not be losing any of the hydrochloric acid (e.g. the measuring cylinder and burette in steps 1 and 2 of my practical procedure) and therefore will not be making slightly lower concentrations. Again when I calculate the solubility of the calcium hydroxide solution I will get a more reliable answer.
I will also only add a few drops of methyl orange indicator so I can clearly observe and colour changes when the acid alkali solution neutralises.
In these ways I believe that when I carry out my experiment I will be ensuring the test is fair and therefore I will obtain more accurate and reliable results.
E.g. Below are shown some of the percentage errors for several of the apparatus I will use and demonstrates the precision of the apparatus I shall use.
250cm3 Volumetric Flask = 0.08%
Burette = 0.2%
Risk Assessment
Details of Sources
1. Salters Advanced Chemistry 2000- Worksheets Elements of Life 2.1 page 4
This was used to help me prepare a procedure for the titration
2. Salters Advanced Chemistry 2000- Worksheets Elements of Life 2.1 page 5
This was used to help me in developing a strategy for the equipment I will use in my titration, as they allow me to see how accurately results can be taken for pipette and volumetric flask.
3.
This was used to help me in developing a strategy and seeing accuracy of burette and other equipment
4. Handout Sheets referring to Health and Safety about:
- Calcium Hydroxide
- Hydrochloric Acid
- Methyl Orange
5. Information Sheet on the use of indicators in Acid Alkali Titrations
Enabled me to see what kind of indicator I will need to use for my experiment- Methyl Orange in my case
Results
Analysis
Ca(OH)2+ 2HCl CaCl2 + 2H2O
Therefore number of moles of HCl = twice the number of moles of Ca(OH)2
Moles of HCl used = Concentration x Volume
(mol) = 0.0300 x 0.025
= 7.5 x 10-4 mol
Therefore moles of Ca(OH)2 used = 7.5 x 10-4 / 2
= 3.75 x 10-4
Therefore concentration of Ca(OH)2 = moles / volume
= 3.75 x 10-4 / 0.02715
= 0.0138 moldm-3 (3 sig. fig)
Evaluation
In my experiment I did not obtain any anomalous results, which demonstrates the precision and accuracy of the experiment. The two real titrations I carried out- i.e. those excluding the rough titration- were within 0.1cm3 of each other and illustrates the reliability of the experiment. The results I obtained were also very accurate as they could be read to 2 decimal places, which demonstrates the precision of the experiment.
Below I have calculated the percentage error for some of the main pieces of apparatus I used and once again shows the accuracy of the instruments.
Percentage Error = (error x 100) / reading
Percentage error for a
250cm3 volumetric flask = (0.2 x 100) / 250
= 0.08 %
Percentage error for a
25cm3 pipette = (0.06 x 100) / 25
= 0.24 %
Percentage error for a
50cm3 burette = (0.05 x 100) / 27.15
= 0.18% (2 d.p)
It is therefore clear that despite the very small percentage error I obtained for each piece of apparatus, the most inaccurate piece of apparatus was the pipette I used. However the combined percentage error for this experiment is only about 0.5%, which is very small. This means that the actual volume of calcium hydroxide required to neutralise 25cm3 of hydrochloric acid could go out either way by 0.5%, which equates to just over 0.1cm3 either way of 27.15 (i.e. 27.05- 27.25). This is still a very small cumulative percentage error for the whole experiment and therefore emphasises the precision of the apparatus.
Despite the fact there is an error of approximately 0.5% in my readings I do not believe this is sufficient enough to question the reliability of the experiment. The concentration I obtained for the calcium hydroxide was 0.01381 moldm-3 (4 sig. fig) If I now take into account that the volume of calcium hydroxide needed was 27.05 instead of my actual reading of 27.15, the concentration of the calcium hydroxide would be 0.01386 moldm-3 (4 sig. fig). The difference in concentrations is therefore approximately 0.00005 moldm-3. It is therefore clear that the percentage error is not hugely significant to the reliability of the experiment.
Despite this the experiment could be improved as to obtain more reliable results. The main source of error for this experiment was most probably observing the colour change at which the two chemicals had neutralised. The reason for this being such a large source of error is due to the experimenter having to decide at which point the colour change is sufficient to say the two have neutralised. In this way it is very easy to go slightly over or slightly under the required colour change. As a result, the reading you obtain from the experiment can be greater or smaller than what it should be. It is therefore difficult to produce consistent results and therefore has a big impact on the experiment.
Another source of error from this experiment may be regarded as reading the burette. If you were to look at the position of the bubble whilst your eyes are not horizontal to it, the value you read will be slightly larger or smaller then what it should be. This is another human error which can give inaccurate results.
Based on the data I think the experiment produced very reliable results, as I obtained two titrations within 0.1cm3 of each other. This therefore means the experimental instruments I used were very accurate, which can be seen by the percentage error calculations on the previous page. I obtained no anomalous results also, which demonstrates this.
In order to improve the experiment I would use a pH meter to judge when the two solutions have neutralised. Doing this would enable me to clearly observe when the two have neutralised. This would be seen when the pH meter reads 7. I would therefore quickly add the calcium hydroxide until the pH was about 6, and then slowly add drops until the meter reads 7. At this point the solution would be exactly neutralised and I would eliminate the fact of having to observe the correct colour change. For this reason I would not go over or under the end-point of the experiment, and would therefore help me produce much more consistent results. My experiment would therefore be much more reliable, as well as the apparatus I used being more accurate.
As well as this I would ensure that whilst reading the burette I would be doing this at eye level. This would ensure that I do not read a slightly higher or lower reading which would cause me to calculate an inaccurate concentration of the calcium hydroxide solution. In this way my experiment would produce more reliable results.
