Concentration of diluted HCl = no of moles = 6.746 x10-4 = 0.0224mol dm3
Volume 0.030dm3
Therefore, there would be 0.0224 moles of HCl in 1.00dm3 of diluted solution.
Volume of 2.00mol dm3 HCl = No. Of Moles = 0.224 =0.01124 dm3 = 11.24 cm3
in 1.00dm3 of diluted solution Concentration 2
As I would not be able to measure 11.24 cm3 of hydrochloric acid exactly using a bulb pipette, as they only measure certain set volumes of liquids. Therefore, I will make the volume 10.00cm3, as 10.00cm3 bulb pipettes are available, and calculate the new concentration of hydrochloric acid.
No of moles of HCl in 10.00cm3 = volume x concentration = 0.010 x 2 = 0.020 moles
Concentration of diluted HCl = no of moles = 0.020 = 0.020mol dm3
Volume 1dm3
Therefore, I will dilute 10.00cm3 of the 2.00mol dm-3 hydrochloric acid, which will have a concentration of 0.020mol dm3.
Dilution Apparatus: 1.00dm3 volumetric Flask, 10.00cm3 bulb pipette, Pipette Filler, 2.00 mol dm-3 hydrochloric acid, distilled water in large container and small bottle.
Titration Apparatus: 25.00 cm3 bulb pipette, pipette filler, 50.00 cm3 burette, conical flask, funnel, Methyl Orange indicator solution, 250.00 cm3 limewater containing approximately
1.00g dm-3 calcium hydroxide, beakers, distilled water, white tile, clamp stand.
Diagram:
I chose to use a volumetric flask, as I believe that it is the most accurate instrument available to me for volumetric measurements for dilution, as it has a tolerance of ±0.006cm3. The pipette is also the most accurate method of volumetric measurement available to me for measuring 10.00cm3 of a liquid, as it has a tolerance of ±0.05cm3. The distilled water is needed in a large and small container, so that the volumetric flask can be filled quickly with the large container first, and then the final small additions of water can be made with the small bottle.
I chose to use a burette for the titration, as this is the most accurate instrument available to me for measuring volumes of reacting solutions, as it has an error of only approximately 0.05 cm3.
I also chose a pipette to measure the volume of limewater, as it is the most accurate instrument available to me, with an error of approximately 0.04cm3.
Indicators
Acid base indicators are very weak acids, or bases’ in which either the unionised acid or base molecule produced by ionisation is coloured. For example, consider an indicator, which is a weak acid, with the formula HIn. At equilibrium, the following chemical equation is established.
The acid and its conjugate base have different colours. At low pH, the concentration of H3O+ is high and so the equilibrium position lies to the left. The equilibrium solution has the colour A. At high pH, the concentration of H3O+ is low and so the equilibrium position thus lies to the right and the equilibrium solution has colour B.
Acid base indicators such as methyl orange, phenolphthalein and bromothymol blue change colour according to the hydrogen ion concentration of solution to which they are added. Consequently, they are used to test acidity and alkalinity. They are used to detect the end point in acid-base titration. In an acid base titration, the base solution is added to the acid until the acid has been neutralised and this is known as the equivalence point of the titration.
I chose to use Methyl Orange as the indicator suitable for my experiment as this is suitable for strong acid with weak alkali titration. Bromothymol blue may also be used and so may phenolphthalein but it’s not really reliable because it does not begin to change colour until about pH 8.
Methyl Orange is a reddish orange liquid within the range 3.0<pH<7.0. This means that it will be reddish orange in the limewater and become yellow at the end point of the reaction between hydrochloric acid and calcium hydroxide i.e. when the solution becomes neutralised.
A white tile is needed to place under the conical flask, to make it easier to tell exactly when the indicator has become transparent. The white tile can also be used to help see the liquid meniscus in the burette when reading the volume from it.
Safety Precautions
Dilution:
-
Rinse a 10.00cm3 pipette with the hydrochloric acid and discard the liquid used.
-
Rinse a 1.00dm3 volumetric flask with distilled water to ensure it is clean. Then pipette 10.00cm3 of 2.00mol dm-3 hydrochloric acid into it.
-
Fill the volumetric flask to just below the measurement line with distilled water from the large container and use 10.00cm3 pipette to add last few drops until it reaches measurement line.
- Replace bung of flask and shake the flask to ensure that the particles of acid are evenly distributed throughout the solution.
- Fill volumetric flask with distilled water, using small bottle, until the bottom of the liquid meniscus lies on the measurement line. Replace bung in flask and shake flask again to ensure the solution is mixed
- Then check meniscus level hasn't changed.
Procedure: Titration:
-
Rinse a 25.00cm3 pipette with the limewater and discard the liquid used.
-
Use the pipette to measure 25.00cm3 of the limewater, reading the volume from the bottom of the meniscus with it at eye level. Make sure there are no air bubbles in the liquid, as this would make the measurement of the volume of liquid inaccurate.
- Then place the limewater in a conical flask (after rinsing it with some limewater) with a dropper full of methyl orange indicator, which should go reddish range.
-
Rinse a 50.00cm3 burette with distilled water and the diluted hydrochloric acid, discarding the liquid used.
- Place the burette in a clamp stand making sure it is vertical. Fill it with the diluted acid using a funnel.
- Take the funnel out of the burette, so that no more drops of liquid can enter the burette.
-
Then lower the level of the liquid to the 0.00cm3 mark, using the tap on the burette. Read from the bottom of the liquid meniscus, with it at eye level. Make sure there are no air bubbles in the liquid in the burette, which would make the volume readings inaccurate.
- Set up the apparatus as shown in the diagram.
- Add the hydrochloric acid to the limewater using the tap on the burette, until the Methyl orange has just become yellow, whilst swirling the conical flask. This ensures that the solution is thoroughly mixed and that the methyl orange indicates the overall pH of the solution.
- Record the volume of hydrochloric acid used, reading the burette scale from the bottom of the liquid meniscus.
-
Repeat steps 4-10 until you get 2 results within 0.10cm3 of each other. The results must be within this range of each other, as this indicates that the 2 results are consistent and reliable. You do not need to rinse the burette in the repeats, but the conical flask should be rinsed as residue from previous trials would contaminate the limewater and make the results il1accurate.
-
Use an average of the two results that are within 0.10cm3 of each other to calculate the exact concentration of the limewater in 1.00dm-3. An average must be calculated from the two consistent results, as finding an average should reduce the error in the results and give a more accurate and reliable result, which is closer to the true value.
Percentage Error
I believe that the instruments I have used have a margin of error.
% Error of pipette = 0.05 x 2 = 0.1 %
10.00
% Error of Volumetric Flask = 0.6 x 100 = 0.06 %
1000
Total % error = 0.16
It is best to do a rough titre before doing the actual ones, to get an approximate result, so that you know approximately when the end point is, so the following titres can be done more rapidly.
Whilst carrying out this procedure, eye protection should be worn as hydrochloric acid is corrosive and irritant and can cause burns.
Once the two consistent results have been obtained, e.g. 35.00cm3 and 35.10cm3, the exact concentration of the limewater solution should be calculated as in the sample calculation, which follows:
(All results taken within an error reading of +/- 0.05)
Average titre = 35.00 + 35.10 = 35.05cm3 of HCI
2
No. Moles HCl = concentration x volume = 0.020 x 0.3505
= 7.01x 10-3
Therefore molar ratio HCl: Ca(OH)2 = 2:1
Therefore No of moles of Ca(OH)2 = 7.01 x 10-3 / 2
= 3.505 x 10-3
Mass of Ca(OH)2 = no. Moles x RMM
= 3.505 x 10-3 x 74
= 0.259g
Concentration of limewater = mass = 0.259 = 1.037 g dm-3
Volume 0.25
Therefore, if the two titres gave the results 35.00cm3 and 35.10cm3of hydrochloric acid, then the actual concentration of limewater would be 1.037 g dm-3.
