Finding the concentration of Limewater (Calcium Hydroxide) in a Titration using Hydrochloric Acid as a reagent

Method: To perform the titration the first thing that needs to be done is to dilute the HCl to a suitable concentration, to increase the accuracy of the results it would be helpful if the volume of the alkali equaled the volume of the acid used, and because we know that the concentration of the alkali is close to 1g/dm which is roughly equal to 0.013mol by using the equation mass = mole * RAM.

Also because the balanced equation is:

2HCl (aq) + Ca(OH)2 (aq) --> 2H20(l) + CaCl2(s)

Using acid that is twice the molar of the alkali should result in equal volumes of each being used. So 0.013 * 2 = 0.026 and taking the molar of the acid down by a factor of one hundred 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 will use is very accurate with the greatest uncertainty being only 2% per drop with the at most with the burette and the most accurate being 0.08% uncertainty with the volumetric flask. However to make sure that this accuracy is maintained all errors must be carefully avoided such as parallax errors occurring when your eye line is not level with the line you are filling equipment up to.

However to maintain this accuracy ALL apparatus must be washed with distilled water before and after use as some will be used more then 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. By placing this in a volumetric flask and topping it up to 250ml with distilled water taking into account the parallax error again, This will have taken the molar down to 0.2M so by repeating this process again the molar will be taken down to 0.02M which is now ready for the titration.

By using the pipette again the collect exactly 25ml of Calcium hydroxide in a conical flask with some phenolphthalein placed in it, 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 place 50ml of the diluted acid in the burette and let it flow out at a fairly slow rate. Since this is only a rough titration it doesn't matter if you put too much acid in however try to keep it accurate. 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 drop will make the difference when it comes to the neutralisation and just one more drop 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 3 times for any accurate results to become apparent. Each time you make a reading you must be careful to note the starting volume of acid in the burette and the end volume making sure there are no parallax errors.

Once you have these results you will need to apply the formula:

V1 * C1 = V2 * C2

Where reagent 1 is the hydrochloric acid and reagent 2 is the calcium hydroxide. The 3 results you have from the experiment should be sorted so that any obvious outliers are ignored and the average taken from the rest, ideally the results should all be within 0.2ml of each other. This number represents V1 and the concentration for this is 0.02mol and so by multiplying these numbers together and dividing this by V2. However you must remember that these volumes must be in dm³ and hence divide the volumes in ml by 1000.

Once you have you're the molar of the calcium hydroxide it will need to be converted in to g/dm? and to do this we will need the relative atomic mass as well. Ca(OH)2 has a RAM of 74.1 so my multiplying this by the molar will give us the mass (in grams) per dm?. To see a full diagram showing all apparatus see below:

Diagram:

Bibliography

I have also used the following books/websites during the writing of my coursework:

Book name / website Author Publisher

Advanced Chemistry for you Lawrie Ryan Stanley Thornes

www.studentcentral.co.uk Randall Stevens N/A

www.chemtutor.com David Wilner N/A

www.chemistry.co.uk Stuart Anderson N/A