To calculate the amount needed to dilute the hydrochloric acid, first I need to find the concentration of Ca(OH)2:
Ca(OH)2 = 1 g dm3
Mr of Ca(OH)2 = 40+ (16 x 2 + 1 x 2)
= 74
1/74 = 0.0135
= 0.01
In the balanced equation it is shown that one mole of Ca(OH)2 reacts with 2 moles of HCL, therefore the concentration should be double:
0.01 x 2 = 0.02
By dividing this concentration by the original it will show me how many times the HCL needs to be diluted:
2.00/0.02 = 100
The acid will need to be diluted x100
Indicator Choice
The indicator I will use to in the titration to determine when the limewater has been neutralised will be Phenolphthalein, due to the fact it shows a dramatic change in colour when neutralisation occurs. Therefore my results should be more accurate.
Safety Precautions To Take Before And During Experiment
- The chemicals used in the experiment are harmful in the following ways:
Calcium hydroxide - This is a skin toxicant, therefore care should be taken and contact between skin and chemical should be avoided.
Hydrochloric acid - Greatly corrosive - Can cause permanent damage to skin and eyes, therefore protective goggle should be worn at all times, and any acid spilt on skin should be immediately washed off.
Phenolphthalein - Not registered as an irritant, however if allowed to enter bloodstream can cause severe effects such as collapse and itching skin rash. Cover any scratches or breaks in the skin prior to experiment.
- When any equipment is not in use or finished with, clear away so as to ensure nothing is broken.
- Any spillages or glass broken should be cleaned immediately.
- Handle all equipment with care so as not to break it.
Method
- First I will clean all the equipment thoroughly so that the experiment is not affected by an extra variable.
Method To Dilute Hydrochloric Acid
This method could be done with a 2500 cm3 volumetric flask, however this equipment is not available so the following method will be used:
- Firstly measure exactly 25ml using the pipette into a 250 cm3 volumetric flask.
- Fill the flask with water, so the meniscus lies directly on the 250 cm3 mark.
- Clean out the pipette with distilled water, so the more concentrated HCL is washed out.
- Take 25ml now from the diluted acid in the volumetric flask using the pipette and pour into another volumetric flask (or use the same one after it has been cleaned thoroughly with distilled water)
- Once again fill the flask up to the 250 cm3 mark.
The HCL is now diluted x100 and ready to be used in the titration.
Method Titration
- Set up equipment for the titration as shown in the diagram above.
- Use the pipette to measure 25ml of the limewater solution and pour it into the conical flask.
- Add small amount of Phenolphthalein Indicator to the limewater, and ensure the amount added each time the experiment is repeated is consistent.
- Place the clamp stand and burette on a lower surface with the funnel in the top so it is much easier and safer to pour in the Hydrochloric Acid.
-
Fill the burette up slowly with HCL, making sure you are eye level so you can see clearly when the meniscus reaches the 0 mark. However it is difficult to do this exactly so I will mark down whether or not there is an error of + /- 0.1 This error will be written down every time the experiment is repeated and then the overall error can be calculated.
- Take out the funnel to ensure no extra HCL drips into burette.
- With the conical flask under the burette carefully and systematically add the same small amount of the HCL to the limewater solution every time, constantly swirling the mixture in the conical flask to ensure the two are mixed.
- When the colour of the mixture in the conical flask appears to be changing, slow down the amount of HCL coming out of the burette so it is only dripping. This way you can measure to a very small amount when it was neutralised.
- As soon as the limewater is neutralised close the burette to stop anymore HCL coming out.
- Take the reading of the amount of HCL from eye level on the burette so it is read accurately. The results should be taken to 2.d.p either; .00 or .05
- Now empty all vessels, rinsing them thoroughly with distilled water. Once this has been done the experiment will be repeated 3 times so that the average can be obtained and to ensure the results are accurate. All results should ideally be concordant and within + / - 0.1 of one another.
Analysis
Now with my results and from these an average results I can now calculate the concentration of the limewater solution.
Firstly I will write the balanced equation:
Ca(OH)2 + 2HCL -----→ CaCl2 + H2O
HCL Ca (OH)2
Volume - 7.6 cm3 Volume - 25 cm3
Concentration - 0.2 mol/dm3 Concentration - Unknown
Calculate the number of moles:
No. of moles = volume x concentration
7.6 /1000 = 0.0076
0.0076 x 0.2 = 0.00152
This is the number of moles of HCL however only one mole of Ca(OH)2 reacts with 2 of HCL so this number needs to be halved:
0.00152 / 2 = 0.00076
With this information I can now calculate the concentration of the limewater solution:
Concentration = Moles
Volume
Volume - 25 / 1000 = 0.025
0.00076 / 0.025 = 0.0304 mol/ dm3
This now needs to be converted to g dm3 so it will need to be multiplied by the Mr of Ca(OH)2:
20 + 2( 16 + 1 ) = 74
0.0304 x 74 = 2.2496
= 2.24 g dm3
I will now identify any sources of error and calculate how this would affect the final result I have obtained. I will find the total errors in the equipment I have used, for example on the burette the smallest reading you can measure is 0.05 cm3, to find the percentage error of this equipment I divide 0.05 by the titre and multiply by 100:
Burette = (0.05 / 7.6 ) x 100 = 0.6578
= 0.66 %
The error on both the volumetric flask and pipette is 0.125%, so to find the overall error I will add these errors and use that percentage to find the error on the final concentration of the limewater solution:
0.66 % + 0.125 % + 0.125 % = 0.91 %
2.24 / 100 = 0.0224
0.0224 x 0.91 = 0.0203
= 0.02
So my error in the final result of the concentration of limewater solution is + or - 0.02 from the final result.
Sources:
http://www.bsieducation.org/Education/14-19/topic-areas/applied-science/images/titration-equipment.jpg
http://www.creative-chemistry.org.uk/index.htm