We can see from this equation that ~0.03mol dm-3 would be the ideal concentration, but it is a more difficult to dilute to this concentration as we do not have the required pipettes, and if we used other apparatus then the accuracy of the solution would decrease.
Method
- First of all, all bags must be placed under a desk or to the side of the room so that no one trips over them.
- Safety goggles and lab coats should also be worn as we are dealing with a strong acid and a strong alkali which can be harmful to the skin and eyes.
- It is also vital to the accuracy of the experiment that you use the same bottle of HCl and the same flask of limewater as each solution could be affected in some way in the experiment, so make a note of which flask (1 or 2) you are using during the experiment.
- Next it is time to set up the apparatus.
- Get a clamp stand and place it on the desk.
- Then obtain all the other apparatus listed on page 1.
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The first thing that should be done is to measure out the required concentration of Hydrochloric acid. We are told the acid is 2 mol dm-3 and we must therefore dilute it. To do this we mix together a specific amount of hydrochloric acid in a 250cm3 volumetric flask, this flask is very accurate, with an accuracy of ±0.05cm3 hence the reason why it is used.
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The best way to do this is to choose around 2.5cm3 of hydrochloric acid, this is useful as we have 5cm3 graduated pipettes at our use and by using pipettes we can ensure the accuracy of the experiment.
- To fill the pipette, attach the safety filler to the top of the pipette making sure to not exert too much force. Then push down on the small circle with an “A” on it directly under the top of the filler and squeeze out all the air so that a vacuum is created.
- Then push down on the circle with the “S” on it next to that to suck up the HCl. Once the liquid has reached the visible line, make sure the bottom of the meniscus is exactly in line with the line on the pipette and then place the pipette over a the volumetric flask and hold down the circle with “E” on it to release the HCl. Next fill the flask with water, when you are getting close to the line on the volumetric flask, get a small teat pipette and add the distilled water in drop by drop again making sure the bottom of the meniscus lines up with the line on the volumetric flask.
- Next it would be best to clean out the burette. Firstly make sure the burette tap is closed, then simply pour a small amount of the substance that will be used in the burette, in this case it is Hydrochloric acid solution. After pouring the HCl into the burette, slowly rotate the burette so that acid goes all around, and then open the burette tap over a sink.
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Next we must pipette out our required amount of diluted HCl solution using a 25cm3 pipette. Once the correct amount is taken out, release the acid into the burette making sure the burette tap is closed and a funnel is used and that the burette is filled below eye level.
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Next we must also add a specific amount of our Limewater solution it this case 25cm3 would be best, into a conical flask. Place a white tile on the bottom part of the clamp stand and then place the conical flask on this tile.
- Now add a small amount (of around 3-4 drops) of Phenolphthalein indicator to the Limewater, so that the solution has gone reasonably purple.
- Making sure the funnel is now removed from the top of the burette we can now start the titration. Open the burette tap and with one hand on the tap, use the other hand to constantly stir the conical flask with a gentle motion.
- Once you see the indicator change colour from purple to colourless close the burette tap and note down the final amount of acid left in the burette. And then work out the difference between the start and end point findings.
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Repeat this around three times so that you end up at least 2 results that are within 0.1cm3 of each other.
References
Class notes
Chemistry 1 textbook by OCR
Chemistry in Context textbook by Hill and Holman
Diagram
Analysis
Using the results that I have noted down from the experiment, I will now be able to find the concentration of limewater in this experiment in g dm-3. The results I have obtained are as follows:
I will use the results from Titration 1 and 2 as they appear to be the most concurrent as they are within 0.1 of each other (the 3rd titration was simply done to ensure the accuracy of my experiment), and to work out the average I simply add 21.60 and 21.50 together and then divide the result by 2: so 21.60 + 21.50 = 43.1 / 2 = 21.55.
From these results I can work out the concentration of the limewater in the experiment.
Primarily using the equation: Concentration = Moles / Volume and this equation can easily be modified to give me the variable I desire.
I have obtained the answer of 0.00862 mol dm-3 concentration of Limewater, but we need to find out the concentration in g dm-3. To convert mol dm-3 we simply multiply the concentration in mol dm-3 by the relative formula mass of the limewater. In this case we will therefore get: 0.00862 x 74 = 0.63788 or 0.64 g dm-3.
So the concentration of Limewater in this experiment is 0.64 g dm-3 which is 0.36 g dm-3 below the “approximately 1 g dm-3 concentration of Limewater” which we are told in the beginning of the experiment.
This result may be below the approximate value due to the fact that there did appear to be quite a lot of solid Ca(OH)2 on the bottom of the large volumetric flask which could mean that the concentration is artificially low due to incomplete dilution. Errors from reading of measuring could also have resulted in the lower than expected concentration of limewater, but since we do not know 100% the concentration of the limewater used (or it may be difficult to judge as some of the limewater may have reacted with the Carbon dioxide in the air which therefore could have affected the results) then we cannot make a valid judgment about errors as my final concentration could well be correct.
