Limewater consists of distilled water and calcium hydroxide. In my experiment I will be titrating a sample of limewater with hydrochloric acid in order to find the concentration of the limewater.
The general equation for a neutralisation reaction is:
Acid + Metal Hydroxide Metal Salt + Water
The word equation for this particular reaction is:
Limewater + Hydrochloric Acid Calcium Chloride + Water + Hydrogen
The balanced symbol equation for this particular reaction is:
Ca(OH)2 (aq) + 2HCl (aq) CaCl2 (aq) + 2H2O (l)
Safety Precautions
While the concentrations used in this experiment are not large, acids and bases are both corrosive, and must always be handled with care. Calcium hydroxide (slaked lime) is mildly corrosive, and long-term exposure can damage your skin. Safety goggles must be worn at all times. Long hair should be tied back.
Any spills or breakages of equipment must be reported immediately and cleared appropriately (i.e. broken glass in the broken glass bin)
Method
Before beginning the titrations I need to dilute the hydrochloric acid to a suitable concentration. This is because its concentration is much higher than that of the limewater so the limewater would be neutralised with a very small volume of hydrochloric acid which would inevitably be very difficult to measure. This would give inaccurate results as an error as small as 0.1cm³ would be a high percentage error due to the small volume used whereas, if a weaker concentration was used, a larger volume would be used and therefore the experiment would yield more accurate results.
To calculate the concentration of hydrochloric acid I should use, I must first calculate the approximate molarity of my limewater sample:
Relative Formula Mass of Ca(OH)2
=40 + 2(16+1)
=40 + 34
=74
This calculation tells me that 74 dm‾³ of limewater solution would be a 1moldm‾³ solution. I know that my limewater sample has an approximate concentration of 1g dm‾³ so its approximate molarity is:
1/74 = 0.0135mol/dm³
I know from the balanced symbol equation that my hydrochloric acid must be twice as concentrated as my limewater so the concentration of my hydrochloric acid will be:
0.0135 x 2 = 0.027 mol/dm³
This would take the molar of the acid down by a factor of one hundred which would work well.
So by diluting the acid twice using a volumetric flask and a pipette, the concentration will change from 2M down to 0.02M, which is more suited to the experiment. The equipment I have chosen is very accurate with the greatest uncertainty being only 2% per drop and the most accurate being only 0.08% uncertainty with the volumetric flask which is extremely reliable and unlikely to affect results in any way.
However, to maintain this accuracy, all apparatus must be washed with distilled water before and after use as some will be used more than once and any residual chemicals will affect the outcome.
- To actually dilute the acid you will need the pipette and the pump to collect exactly 25ml taking into account parallax errors where the level of the 25ml line is at the bottom of the meniscus.
- Place this solution into a 250ml volumetric flask and top it up to 250ml with distilled water. This will have taken the molar down to 0.2M.
- Place a bung on the solution and shake to mix up the solution.
- Take another 25ml by the pipette from this solution and place into a different volumetric flask. Top up to 250ml with distilled water. The molar of the acid has now been taken down by a factor of one hundred making it 0.02M which is now ready for the titration.
- By using the pipette again collect exactly 25ml of the calcium hydroxide and place in a conical flask. Add around 5 drops of phenolphthalein to the solution – the colour of the solution will be a purple/pink colour and when the alkali is fully neutralised the solution will become colourless.
- To start titrating pour 50 ml of the diluted hydrochloric acid into the burette using a funnel and let into run into the limewater solution at a fairly slow rate making sure you swirl the solution constantly. Although this is a rough titration, try to keep it as accurate as possible. When the colour starts to weaken slow the rate even more and stop it completely when it suddenly changes colour, this is the end point. Just one more drop will make the difference when it comes to neutralisation and will make the solution acidic instead. When it comes to looking at the colour you can get a much better picture of what colour the solution is by placing the white tile underneath the conical flask.
- By reading off the side of the burette what volume is left and taking this away from the starting volume (which should be 50ml) will give you the volume of acid required to neutralise the alkali. However, since this is not accurate enough and anomalies do occur the test must be performed a further three times until you have gained results within 0.5ml of one another.
- Any obvious outliers are ignored and the average is taken from the rest ideally being within 0.5ml of each other.
Results of the titrations
Average titre = 34.40 + 34.45 + 34.45 = 103.3 / 3
= 34.43cm³
Conclusion
Ca(OH)2 (aq) + 2HCl (aq) CaCl2 (aq) + 2H2O (l)
- Number of moles of HCl:
n = Mv/1000
34.43/1000 x 0.02
= 6.886 x 10 moles
-
Number of moles of Ca(OH)2 (calcium hydroxide)
Reacting ratio = 1 Ca(OH)2 : 2 HCl
6.886 x 10 / 2
= 3.443 x 10 moles
- Concentration of limewater in mol/dm³:
1000 x 3.443 x 10 / 25
= 0.013772 mol/dm³
- Concentration of Limewater in g/dm³:
Formula mass = 74.1
74.1 x 0.013772
= 1.0205052
=1.02 g/dm³ (2dp)
I can conclude that my results are quite accurate as it is very near to the 1gdm‾³ estimate given to us at the beginning of the experiment. I am satisfied with the results that I obtained but will need to go through any source of error. Also, the fact that my titration results are consistent with each other proves that they are reliable.
Evaluation
I felt that the practical went well and I selected appropriate apparatus for the task given. I achieved consistency within my results because they were all within 0.05cm³ of each other.
The apparatus that I used: the pipette, volumetric flask and burette all have an accuracy of approximately + or – 0.05 cm³ so any errors occurring through equipment is minimal yet still has to be taken into consideration.
I rounded my end result up to 2 decimal places because this represents the reliability of the equipment which is also accurate to 2 decimal places therefore further digits are not required.
I felt there were few limitations in the procedure and any limitations there was were minimal. An example of this is when the solution during the titration gets on the walls of the conical flask which may stop it from being neutralised and slightly disturb the experiment. This however can be helped if you swirl the solution around thoroughly during the titration to make sure that it is all mixed together.
Some of the errors were likely to have come from measurements. For example a source of error could be a parallax error which involves the meniscus present in the burette and pipette. Another form of human error could be the judgement of colour change in the titration when it reaches neutralisation. The human eye could be wrong which would results in a slight error of results.
None of these sources of error could really be improved dramatically. However, if I were to use a larger quantity of limewater for the titrations this would reduce the percentage error. Also, by repeating the experiment several more times would give me a more reliable and accurate set of results from which I could have obtained a better average.
References:
- Advanced Chemistry for you text book
- Chemistry 1 text book endorsed by OCR