CaO(S) + H2O (l) Ca(OH)2(S)
- Dissolving the solid calcium hydroxide with water produces the aqueous limewater solution.
Ca(OH)2(S) + H2O (l) Ca(OH)2(aq)
Equations for the experiment
For the reaction I will be reacting limewater with hydrochloric acid to determine the concentration of the limewater. The reaction between the limewater and hydrochloric acid will produce calcium chloride and water, and the equation for this reaction is as follows: -
Ca(OH)2(aq) + 2HCl(aq) CaCl2(aq) + 2H2O(l)
For this experiment I will use the following apparatus: -
- Goggles
-
50 cm3 Burette
-
25 cm3 Pipette
-
250 cm3 Conical flask
-
250 cm3 Volumetric flask
- Clamp and Stand
- White tile
- Funnel
-
250 cm3 of Limewater
-
2.00 mol/dm-3 of standard solution hydrochloric acid
- Methyl Orange indicator solution
- Distilled water
Method
Since the concentration of HCl is too high for the experiment I will dilute it by 98% from 2.00 to 0.04 mol/dm-3 using distilled water: -
-
I will take 5 cm3 of 2.00 mol/dm-3 HCl and put it into a 250 cm3 volumetric flask using a pipette
-
Then I will add 245 cm3 of distilled water, which will produce 250 cm3 of
0.004 mol/dm-3 HCl.
Titration: -
- First I will setup the clamp and stand securely then place the burette into the clamp
-
I will put 50 cm3 of 0.004 mol/dm-3 HCl into a burette using a funnel
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I will then put 25 cm3 of limewater into a conical flask using a 25cm3 pipette.
- Then I will put three drops of methyl orange indicator solution into the conical flask containing the limewater. When the methyl orange is added the colour of the limewater will be light orange, which should turn pink/purple by the end of the titration.
- After which I will place the conical flask on the stand under the burette.
- I will then titrate the HCl into the limewater in the conical flask until the limewater changes colour from light orange to pink/purple
- When the colour changes I will stop the burette and take a reading of the burette
To gain accurate and consistent results I will repeat the experiment at least three times.
Fair test
To keep the experiment fair and to gain accurate results I will dilute the HCl by 98% as I have stated before. To reduce errors I have chosen many accurate apparatus such as the volumetric flask, pipette and the burette all of which have error margin of ±0.05 cm3, which should produce suitable and accurate results. I have chosen methyl orange as my indicator because it has an end point of ph 8-5, which will be suitable for limewater, a relatively weak base, and HCl, a strong acid. The methyl orange will show the end point when the acid and base chemicals break up and not before which would happen with phenolphthalein.
Safety
When conducting any experiment safety is of the utmost importance and requires everyone to follow the safety procedures carefully and correctly, ensuring everyone who handles any chemicals wears eye protection. All long hair should be tied back and extra care should be taken when working with corrosive materials such as acids. I will be wearing the following safety equipment during the experiment: -
- A lab coat – to protect my body from any highly concentrated chemicals
- Goggles – to protect my eyes from any spilled chemicals
- Gloves – to protect my hands from corrosive acids
Implementation
For this experiment I was to be given 2.00 mol/dm-3 HCl instead I was given
1.89 mol/dm-3 HCl and therefore when I diluted 5 cm3 of 1.89 mol/dm-3 HCl in 245 cm3 of distilled water it produced 250 cm3 of 0.0378 mol/dm-3 HCl. Other than that the rest of my experiment went according to plan and I believe the results I have obtained are fairly accurate.
Results
Average Titre = 09.20 + 09.20 +09.10 = 09.17 cm3
3
Calculations
Moles of HCl:
mol. = conc. (HCl) vol. (HCl)
1000
= 0.036 9.17
1000
= 0.00033012
= 3.3 104 mol.
mol. of Ca(OH)2:
Ca(OH)2(aq) + 2HCl(aq) CaCl2(aq) + 2H2O(l)
1 mol. of Ca(OH)2 reacts with 2 mol. of HCl.
∴ mol. of Ca(OH)2 = mol. of HCl ÷ 2 = (3.3 × 104) ÷ 2 = 0.00016506
= 1.65 × 104 mol.
Concentration of limewater in mol/dm-3:
vol. of limewater in dm3 = 25 cm3 ÷ 1000 = 0.025 dm3
= 2.5 × 10-2 dm3
conc. = 1.65 × 104 mol. ÷ 2.5 × 10-2 dm3 = 6.6 × 10-3 mol/dm-3
Concentration of limewater in g/dm-3:
rfm of Ca(OH)2 = 40.1 + (16+1) × 2 = 74.1g (rfm = Relative Formula Mass)
conc. of limewater = 74.1 × (6.6 × 10-3) = 0.48906 g/dm-3
= 0.49 g/dm-3
Conclusion
I think that results that I have acquired are fairly accurate. I am satisfied with my results and the fact that they are consistent. Although I feel my results are fairly accurate there were some errors in the experiment that could not be prevented.
Evaluation
The apparatus and equipment that I selected for this titration were quite reliable and any errors from these instruments are small. Other errors during the experiment that I could not have prevented are the small splashes of solution in the conical flask and the pipette holding a drop or two of limewater may have affected my results although I believe these errors to be too small to be of any relevance. The errors that I believe to have a serious affect on my results are human errors on measurements; the meniscus makes it very difficult to take an accurate reading of the burette and the pipette. Another human error occurs while judging the colour change of the solution; it is virtually impossible to judge the colour change of the limewater exactly the same all three times with the human eye.
This experiment can be greatly improved if a computer was used to take all the measurements and repeating the experiment more, the more the better, to get a better average titre and overall more reliable and accurate results.
I feel that my results are fairly accurate and reliable and this is proven by the fact that I obtained two identical results and third was 0.1cm3 of the mark.
