Acid + Metal Hydroxide = Salt + Water, so the balanced equation for the reaction is-
Ca(OH)2(aq) + 2HCl(aq) CaCl2 + 2H2O(l)
Limewater + Hydrochloric acid Calcium chloride + water
So that I can make my experiment accurate and reliable I will have to dilute the hydrochloric acid significantly. I think that if I diluted my hydrochloric acid to 0.1 mol dm3 it would enable me to achieve reliable and accurate results, to dilute it to this value the dilution factor would be 20. I will make 500 cm3 of dilute hydrochloric acid and I will use 25cm3 of limewater to do the titration.
Method
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First I will dilute the hydrochloric acid. I will dilute the HCl to 0.1 moles, so I will pipette 25 cm3 of HCl into the volumetric flask.
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Then I will add 475 cm3 of distilled water in the same volumetric flask. I will measure the distilled water by filling the volumetric flask to the graduated mark on the volumetric flask which is the 500cm3 mark. This would give me 475 cm3 of distilled water.
- Then I will invert the volumetric flask a few times so that the solution is mixed well.
- I will then set up my apparatus which is shown in the diagram.
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I will add 25cm3 of limewater to the conical flask using a measuring cylinder.
- Then I will add 5 drops of methyl orange indicator to the limewater so that I can see when the neutralisation point is obtained. The solution will turn light orange and should turn pink/red by the end of the titration.
- I will then wash out the burette with HCl to make sure it isn’t contaminated which would affect the results.
- Then I will fill the burette with the diluted HCl and note the amount of HCl in the burette.
- I will then begin to let the HCl drip into the limewater, it is important to let the HCl drip slowly into the limewater otherwise the neutralisation point will pass too fast and you will miss the neutralisation point.
- When the solution is neutralised I will record the amount of HCl remaining in the burette. Then I can subtract the amount left from the amount of HCl I started with.
- I will then note down the amount of HCl used to neutralise the limewater solution. I will repeat the experiment 3 times in order to obtain consistent and reliable results and record all the results.
To reduce the amount of errors in my experiment I have chosen accurate and reliable equipment, so that is why I have chosen the volumetric flask, pipette and burette because all of these instruments are accurate and reliable. The volumetric flask has an accuracy of ±0.2ml, the burette and conical flask have an accuracy of ±0.5ml which is suitable for my experiment and should produce accurate and reliable results. 2
I had to be careful when choosing my indicator because the limewater being used will be weak based so I will have to use methyl orange which has a colour changing point on the pH scale between 3-5 whereas phenolphthalein indicator has a colour changing point which is much higher up the pH scale, because of this, if I use phenolphthalein then the colour change would be reached before the chemicals acid bases breaks up. 3
To be safe when doing the experiment I will use eye protection because I will be working with acid. I will also be very careful when handling the chemicals because they are corrosive.
To find out the concentration of the limewater solution-
I will take the average titration which is the 3 titrations added up and divided by 3.
Then I will work out the number of moles of HCl used-
1dm3 = 1000cm3
Number of moles = concentration × volume
0.1/1000 × average titration = number of moles of HCl used
Number of moles of Ca(OH)2
Reaction ratio = 1 Ca(OH)2 : 2 HCl
(Number of moles of HCl used) ÷ 2 = Number of moles of calcium hydroxide
Concentration of limewater: dm3
25.0 cm3/1000 = 0.025 dm3
(No of moles of calcium hydroxide) ÷ 0.025 = concentration of limewater (mol dm3)
Concentration of limewater in g dm3
Formula mass Ca(OH)2 = 40.1 + (16+1) × 2 = 74.1g
Concentration of Ca(OH)2 in g dm3 =74.1 × (concentration of limewater in mol dm3)
Diagram
987 words
References
1
What is limewater.(2003).Randy Holmes-Farley.URL:http://www.animalnetwork.com/fish2/aqfm/1999/aug/bio/default.asp
2
Volumetric flaks.(2003).Dartmouth college.URL:http://www.dartmouth.edu/chemlab/techniques/volumetricflasks.html
3
What is methyl orange? How is it made?.(2003).Fred Senese.URL:http://antoine.frostburg.edu/chem/senese/101/acidbase/faq/print-methyl-orange.shtml
