I chose to use 30.00cm³ of limewater for each titre because it is a large enough amount to get a reasonable degree of accuracy and it is also possible to measure out using a 30.00cm³ glass pipette.
I will be using the following apparatus in my procedure:
Dilution Apparatus:
500.00cm³ conical flask
250.00cm³ volumetric flask
1.00cm³ glass pipette
50.00cm³ glass pipette
Pipette filler
2.00 molar hydrochloric acid
Titration Apparatus:
30.00cm³ bulb pipette
Pipette filler
50.00cm³ burette
25.00cm³ conical flask
Funnel
Phenol phthalein indicator solution
250.00cm³ limewater containing approximately 1.00g dmˉ³ calcium hydroxide
2 × 250.00 cm³ beaker
Distilled water
White tile
Clamp stand
Filter paper
Goggles
300.00cm³ dilute hydrochloric acid
Litmus paper
Safety Precautions:
Calcium Hydroxide can cause burns and is corrosive. It is also an irritant to the eyes, skin, and respiratory system when the vapour is inhaled. Due to this it is imperative that eye protection is worn at all times.
Hydrochloric acid can also cause burns as well as the vapours being very irritating to the respiratory system. It is also an irritant to the skin and eyes. Hydrochloric acid also reacts very strongly with certain metals such as calcium and must not be kept or stored any where near them. Due to these factors, goggles must be worn at all times. It is also advisable to wear a lab coat and latex gloves. Caution should also be taken when handling hydrochloric acid, not to inhale deeply.
Factors that should be taken into account:
There are also certain factors that should be accounted for during the procedure. It is important to keep the temperature of the surroundings steady, and as close to the ideal room temperature (21°C) as possible. This is because the temperature of the room can cause liquids to expand or shrink in size, and this can affect the accuracy of the experiment. This will help to keep the volume of the limewater constant at 30.00cm³. It is also important to note the initial and final volumes of hydrochloric acid for each titre accurately so as to keep error down to a minimum.
Dilution:
It is important to dilute the hydrochloric acid to a suitable strength to allow for a more accurate experiment. I will be diluting the hydrochloric acid to a 0.020 molar solution so that it is twice the strength of the limewater. I have explained the reasoning for this in my calculations. I want to obtain 300.00cm³ of the dilute hydrochloric acid, at a ratio of HCl:H2O of 1:99. This will give me the strength of dilute hydrochloric acid that is needed in the procedure.
To obtain a 0.020 molar solution of hydrochloric acid I firstly cleaned all my apparatus with distilled water, and then acetone to get rid of any residue. I then measured 3.00cm³ of hydrochloric acid into a 250.00cm³ volumetric flask using a 1.00cm³ glass pipette 3 times. I then filled up the flask to the 250.00cm³ line with distilled water so that I had 250.00cm³ of dilute hydrochloric acid solution. I then put a bung into the volumetric flask and shook it to ensure that the solution was completely mixed. However, I needed 300.00cm³ of solution so I poured the 250.00cm³ that I already had into a 500.00cm³ conical flask and then added another 50.00cm³ of distilled water using a 50.00cm³ glass pipette. I then had 300.00cm³ a 0.020 molar solution of hydrochloric acid.
Titration:
I started my procedure by making sure that all of my equipment was clean, by first rinsing everything with distilled water and then using acetone to ensure that there was no droplets of water inside any of the equipment that could affect the accuracy of the titration. I then waited for the acetone to evaporate to make sure that it was clean.
I measured some limewater using a 30.00cm³ glass pipette, making sure that there were no bubbles in the limewater, and also made sure to read the meniscus at eye level to ensure that the reading was entirely accurate. I then placed the measured limewater into a 250.00cm³ conical flask, before adding 5 drops of the indicator solution (phenolphthalein). Once that was done I placed the conical flask on a white tile under where the burette was to be placed. I then flushed the burette with hydrochloric acid to get rid of any bubbles, but making sure that I left some hydrochloric acid in the burette so that no more bubbles accumulate. I then made sure that the burette was vertical, placed it in the clamp and then topped up the burette with hydrochloric acid using a funnel so that none was spilt. I then made sure to remove the funnel so that no more hydrochloric acid could drop into the burette and alter the amount in the burette after I read the measurement which could affect the accuracy of the titration. I then placed a piece of filter paper behind the burette to aid in the reading of the meniscus. This allowed me to read it more accurately because it removed the distractions in the background. The meniscus was at 0.45cm³, making sure to read it at eye level to ensure the reading was entirely correct. The apparatus I used is shown how it was set up in the diagram below:
I made sure that the tip of the burette was slightly inside the conical flask so that no hydrochloric acid could escape the flask by splashing outside of it, and cause the titration to have inaccuracies.
The dilute hydrochloric acid was then added to the limewater using the tap on the side of the burette to allow the dilute hydrochloric acid to be let out. I did this until the phenolphthalein turned clear. The whole time, I made sure that I was constantly swirling the conical flask to make sure that the solution was mixed evenly. Once the solution had become clear I read the meniscus on the burette at eye level, and then recorded the result in a table. The first titration was a ‘pilot’ because I just did a test run to get an idea of how much dilute hydrochloric acid. This will allow me to do the other titres faster.
I then repeated the whole process until I got two results within 0.10cm³ of each other. When doing the other titrations I had to make sure that as soon as the phenolphthalein started to go clear I added the dilute hydrochloric acid drop by drop to ensure that I was not using too much, which would allow me to get the best result when calculating the exact concentration of the limewater solution.
Once each titration was complete I used litmus paper to check that the pH was neutral at pH7, this ensured that the correct amount of hydrochloric acid was being used to neutralise the limewater. It was also important to clean the conical flask in between each titre to make sure that there was no residue of the previous titre left that could affect the result of the following titre.
I obtained the following results from the whole procedure:
Results:
Table to show the amount of 0.020 molar hydrochloric acid required to neutralise 30.00cm³ of limewater with an approximate strength of 1g dmˉ³.
When calculating the exact molarity of the limewater I will be using an average of my 4 results to give a better figure that will be more reliable and therefore, will give me a more accurate calculation. I will not be including the pilot in this because it was merely a test run and I do not feel that I did it to the same accuracy as the others. I did the pilot titration to get an approximate result that would allow me to do the other titrations more rapidly, due to knowing roughly where the end point is. This is why the pilot titration is larger than the others.
Calculations:
Average titre = 38.90 + 38.70 + 38.50 + 38.75 = 38.7125cm³ of 0.020 molar HCl
4
To work out the exact molarity of the limewater, the following calculations must be made:
Number of moles of HCl = concentration × volume
Number of moles of HCl = 0.020 × 0.387125
Number of moles of HCl = 0.0077425
Number of moles of HCl = 7.7425 × 10ˉ³
Due to the molar ratio of HCl:Ca(OH)2 = 2:1
7.7425 × 10ˉ³ = 3.87125 × 10ˉ³
2 .
Mass of Ca(OH)2 = number of moles × RMM (Relative Molecular Mass)
Mass of Ca(OH)2 = (3.87125 × 10ˉ³) × 74
Mass of Ca(OH)2 = 0.286g
The concentration of limewater is therefore 0.286 × 4 = 1.144g dmˉ³
Therefore the molarity of the limewater = 1.144 = 0.015 moles
74
Percentage Error:
Due to there being small margins of error in volume of each different apparatus it is necessary to explain and calculate the possible percentage of error. In the below list I have included the name of the apparatus and the margin of error that the apparatus has. However, I have only included the margin of error for the apparatus that I used to do measurements with because it does not matter for the other equipment that I used. I will also be separating the equipment into two categories; dilution and titration. This is to outline what percentage of error occurred when, to give an idea of where it was.
Dilution:
1.00cm³ glass pipette – margin of error = ± 0.01cm³ × 3 = ± 0.03cm³
50.00cm³ glass pipette – margin of error = ± 0.10cm³
250.00cm³ volumetric flask –margin of error = ± 0.15cm³
Therefore the total margin of error in the dilution that can be accounted for is:
0.03 + 0.10 + 0.15 = ± 0.28cm³
Titration:
30.00cm³ glass pipette – margin of error = ± 0.09cm³
50.00cm³ burette – margin of error = ± 0.10cm³
Therefore the total margin of error for each titration is:
0.09 + 0.10 = ± 0.19cm³.
Due to the overall margin of error being so small, that even adding up the dilution and the titration you only get ± 0.47cm³ which isn’t even a large enough figure to round up to 1cm³. This sows that the margin of error is so small that it is not even worth taking it into account due to the figure being so insignificant in the scheme of things.
Conclusion and Analysis:
From this I conclude that the exact strength of the limewater is 0.0154 moles. This is a relatively close number to 0.0135 which was the original estimate of the strength. Due to this I find that my methods were pretty reliable under the conditions and apparatus that I was using. However, after conducting the investigation I found out that a more suitable indicator to use would have been methyl orange or bromocresol green due to acid – base equilibria. Either of these two indicators would have been more suitable to the particular titration that I was doing, because they are more suited to a titration between a strong acid and a weak alkali like hydrochloric acid and limewater. This is shown why in the diagram below:
Diagram to show the change in pH when a strong acid is added to a weak alkali:
In the diagram it is obvious that for a strong acid and a weak base, the methyl orange is a much more suitable indicator because you want to use an indicator where point that it changes colour is as close to the equivalence point as possible, and is on the steep part of the curve.
As you can see from this diagram, phenolphthalein is not a very good choice. However I realised this whilst doing my pilot experiment and tested the pH of each titre once it had completed to ensure that the solution had been neutralised. I have shown another diagram below of the changing points of phenolphthalein and methyl orange but this time they are also compared to litmus to show how it is useful to use it in conjunction with one of the other indicators to give a more accurate neutralisation:
Diagram to show the point where different indicators change colour in relation to pH levels:
As you can see from above, the litmus changes over an unusually wide range that spans over pH7, which is why it allows you to get a more accurate neutralisation.
Phenolphthalein is the indicator that I used. Below I have included another small diagram to show why it changes colour. Phenolphthalein is another weak acid, as are all indicators.
In this case, the weak acid is colourless and its ion is bright pink. Adding extra hydrogen ions shifts the position of equilibrium to the left, and turns the indicator colourless. Adding hydroxide ions removes the hydrogen ions from the equilibrium which tips to the right to replace them - turning the indicator pink. The half-way stage happens at pH 9.3. Since a mixture of pink and colourless is simply a paler pink, this is difficult to detect with any accuracy. This is another reason why I used the litmus paper to ensure that my results were accurate.
Other than using a more suitable indicator, I am generally pleased with the results I obtained and feel that the exact molarity that I found out for the limewater is very plausible. I am not saying that I was completely accurate. However, I do feel that with the equipment and apparatus provided I could not have obtained significantly better results.
Bibliography:
– Search engine
– Online encyclopaedia
Chemistry 1 – Textbook by: Brian Ratcliff, Helen Eccles, David Johnson, John Nicholson, John Raffan.
Advanced Chemistry for You – Textbook by Lawrie Ryan.