If these steps are followed, the titration should be as accurate as possible, given the margin for error in reading values from the burette and volumetric flask.
Safety Precautions
To ensure the experiment is carried out safely, certain safety procedures should be adhered to.
HYDROCHLORIC ACID – Corrosive, Irritant
Wear eye protection and gloves at all times when using HCl
If spilt in eyes: Immediately rinse eyes with clean, distilled water for at least 15 minutes. Lift eyelids to ensure eyes are thoroughly flushed. Do not attempt to neutralise the acid. Seek medical opinion.
If spilt on skin: Immediately wash the skin for 10 minutes to remove all acid. If the skin is still irritated, seek medical advice. Keep affected area cool.
Is ingested: Do not induce vomiting. Give large quantities of water if victim is still conscious. Keep warm.
If spilt on clothes: Remove affected clothing, as corrosive acid may burn through clothing and cause harm to skin.
CALCIUM HYDROXIDE – Slightly corrosive
Wear eye protection.
If spilt in eyes: Immediately rinse eyes with clean, distilled water for at least 15 minutes. Lift eyelids to ensure eyes are thoroughly flushed. Do not attempt to neutralise the acid. Seek medical opinion.
If spilt on skin or clothes: Wipe of affected skin, then wash in tepid water, remove any contaminated clothing.
Calculating the concentration of Calcium Hydroxide
To calculate the concentration of the Ca(OH)2, a mean average value of the acid added must be found.
The amount in moles of Hydrochloric acid must be found:
Amount in Moles = amount used in dm-3___
Relative Molecular Mass HCl
Going back to the equation of the reaction:
We can see that for every mole of Ca(OH)2, there are 2 moles of HCl. This means that there will be half the number of moles of Ca(OH)2 as there are of HCl.
The amount of Ca(OH)2 used was 25cm3. This means that the concentration can be found from:
Concentration = Amount in moles__
Amount used in dm-3
This value is the amount of Calcium Hydroxide present in the saturated solution used.
Results & Analysis
* The mean average acid solution used does not include the rough titration, or the first repeat, which was clearly an anomaly.
My results show that the mean average volume of acid used was 37.20cm3.
The concentration of this acid can be found by a series of steps, based on these results:
Original concentration of acid was 0.3M. This was reduced when the acid was diluted.
25cm3 HCl + 225cm3 distilled water 250cm3 hydrochloric acid solution
Concentration of HCl used was 0.03 mol dm-3.
I can also find the concentration of the Ca(OH)2 used in the experiment:
Ca(OH)2 (aq) + 2HCl (aq) CaCl2 (aq) + 2H2O (l)
25cm3 + 37.20cm3
Concentration 0.03mol dm-3
Unknown
I can find the number of moles of HCl used by the following method:
Moles = Concentration (mol dm-3) * Volume (dm-3)
= 0.03 * 0.0372
= 0.001116 moles
This means I have used 0.001116 moles of HCl to neutralise 25cm3 of Ca(OH)2 solution.
NOTE: At this stage, it is important to use full answers and not to use significant figures, because such rounding can make a significant difference to the final concentration of Ca(OH)2.
The equation of the reaction tells me that there are 2 moles of HCl used for each one of Ca(OH)2. Therefore, the amount of moles of Ca(OH)2 used is half the amount used of HCl:
0.001116 = 0.000558 moles
2
This figure is the amount of moles used in only 25cm3 of Ca(OH)2 solution. I need to find the concentration (amount in 1000cm3).
This can be achieved by multiplying the value by 40, to find the amount of moles in 1000cm3, ie. the concentration:
0.000558 * 40 = 0.02232 moles.
This is the number of moles in one litre, or one decimetre (dm-3), and can be written in the common form of:
0.0223mol dm-3 (3 significant figures)
Calculating Percentage Error
In the experiment, there were 4 sources of potential error, all of which are due to the scale of the apparatus I have used:
- Error in reading pipette of HCl
- Error in filling volumetric flask
-
Error in reading pipette of Ca(OH)2
- Error in reading burette
The percentage error is the sum of the potential error in each of these instruments, and can be found by the following calculation:
Error =_Potential error_ * 100
Value obtained
Pipette of HCl: _0.25_ * 100 = 1%
25.0
Volumetric flask: _0.25_ * 100 = 0.1%
250
Pipette of Ca(OH)2: _0.25_ * 100 = 1%
25.0
Reading burette: _0.05_ * 100 = 0.134%
37.20
Total percentage error = 2.23% (3 significant figures)
Of these errors, I believe that the potential error in reading the pipette is the most significant, because it has the highest individual error, and there was only a single line for measuring the volume, making it hard to assess the margin for error when using the pipette.
Evaluating Evidence
In my experiment, I found one anomalous result. This clearly did not fit in with the pattern, and as such it was not included in the average result I calculated.
Another source of potential error is when using the pipette, it is possible that air bubbles could enter the pipette. Whilst I tried to identify and eliminate these air bubbles, I cannot rule out the possibility that one of these air bubbles resulted in less Ca(OH)2 being included in the titration. This would account for the anomalous result I obtained, which showed a lower concentration than expected.
I believe that overall, my evidence was accurate and reliable because I have found 3 values which agree to within 1 decimal place. The apparatus I have used was the most accurate available to me, and the single anomalous result I obtained can possibly be accounted for by an air bubble. The apparatus was all transparent glassware, so I did not have any difficulty reading the endpoint on the burette, or the line on the pipette.
To improve the accuracy of the procedure, I could have repeated the experiment more times, or used different concentrations of acid, and different ratios of acid to alkali. Hopefully, these would all have given the same concentration of calcium hydroxide each time.
This experiment would improve or verify the accuracy of the results, because it would be necessary to dilute the hydrochloric acid to a different concentration, which would show whether I had made any errors when diluting the HCl whilst gathering my evidence. If such an error had occurred, it would not show on my results, because I had made 250cm3 of diluted 0.03M hydrochloric acid at the start which was used throughout the experiment.