0.3375
As I plan to use a 50cm3 burette, an ideal volume of HCl to be measured out would be between 20-30cm3; so if I want to use about 25cm3 of HCl then the concentration will need to be about:
0.000746 = 0.02984 mol dm-3
25/1000
To make dilution process easier (and more accurate) I will dilute to a concentration of 0.02 mol
dm-3 .I could use a single or two fold dilution.
Dilution Method
If a single fold dilution was used:
equipment:
- 50cm3 burette
- burette stand
- 250cm3 volumetric flask
- distilled water
- 2 mol dm-3 hydrochloric acid
Begin with 2.00 mol dm-3 HCl, take 2.5cm3 using a 50cm3 burette and put it into a 250cm3. Add 222.5cm3 of distilled water, using the 250cm3 mark on the volumetric flask.
50cm3 burette:
(accurate to ± 0.1cm3) 0.1 × 100 = 4 %
2.5
250cm3 volumetric flask: 0.15 × 100 = 0.06%
(accurate to ± 0.15) 250
sum of measurement errors: 4 + 0.06 = 4.06
Using a two fold dilution:
equipment:
- 25cm3 pipette
- pipette filler
- 100cm3 volumetric flask
- 250cm3 volumetric flask
- distilled water
- 2 mol dm-3 hydrochloric acid
Begin with the hydrochloric acid at 2.00 mol dm-3. Using a 10cm3 pipette measure 10cm3 of the 2M HCl into a 100cm3 volumetric flask then add 90cm3 of distilled water, and shake to mix; this will give 100cm3 of 0.2cm3 HCl. Then take 25cm3 of the 0.2M acid that has just been made and – using a 25cm3 pipette and pipette filler – put it into a 250cm3 volumetric flask with 225cm3 of distilled water and shake; this will provide 250cm3 of 0.02 mol dm-3 hydrochloric acid to use in the main experiment.
10cm3 pipette:
(accurate to ± 0.04 cm3) 0.04 × 100 = 0.4 %
10
100cm3 volumetric flask:
(accurate to ± 0.20 cm3) 0.2 × 100 = 0.2 %
100
25cm3 pipette: 0.06 × 100 = 0.24 %
25
250cm3 volumetric flask:
(accurate to ± 0.15) 0.15 × 100 = 0.06
250
sum of measurement errors: 0.4 + 0.2 + 0.24 + 0.06 = 0.9 %
The sum of the measurement errors for the two fold method are much smaller so I think this will be the simpler and more accurate method, so I will use the two fold dilution method (outlined above).
In order to know when the alkali has been neutralised I will need to use a suitable indicator. Two indicators that function within this pH range are phenol phthalein and bromothymol blue; I have decided that phenol phthalein will give me more accurate results as it produces a more definite end point (it changes from vibrant pink to clear in acid, whereas bromothymol blue is blue in alkaki and yellow in acid but has a halfway point of green so it would be more difficult to determine an exact point).
Titration Equipment List And Diagram
- 50cm3 burette
- burette stand
- 25cm3 pipette
- pipette filler
- funnel
- 250cm3 conical flask
- white tile
- 0.02 mol dm-3 hydrochloric acid
- 1g dm-3 calcium hydroxide solution
- phenol phthaline
I am using volumetric equipment (pipette
and burette) so that the measurement errors are lower
and my end result is more accurate. The white tile
is used so that the colour of the alkali and indicator
mixture is more visible and I will be able to determine
the end point more easily.
Titration Method
Set up the equipment as shown in the diagram: attach the burette to the 50cm3 burette stand at a height that allows the burette to be read easily and leaves enough room for the conical flask to be placed underneath Using a plastic funnel, fill the burette up to the 0cm3 mark with the 0.02 mol dm-3 hydrochloric acid made during the dilution process. Remove the funnel so excess drips of acid do not affect the initial volume. Use a 25cm3 pipette and pipette filler to put 25cm3 of the 1g dm-3 calcium hydroxide solution in a 250cm3 conical flask. Add a few drops of phenol phthaline so that the pink colour is clearly visible. Place the conical flask on top of a white tile and underneath the nozzle of the burette. Allow acid to run into the conical flask until a colour change – from pink to colourless – occurs, swirling the flask as the acid runs out to ensure it is properly mixed. Once a rough result has been obtained, repeat process, but as approximate end point approaches slow down the flow of acid and swirl the conical flask more often. Do a minimum of 3 repeats, so that at least 3 of the results are within 0.01 of each other; this will ensure the average result is accurate.
(Essential AS Chemistry for OCR pg 23)
Results Table
Set out a results table like the one below:
mean change in volume:
By putting the final burette reading above the initial one it is easier to subtract the two. The mean volume given will be the amount of hydrochloric acid needed to neutralise the limewater; it will then be possible to work out the exact concentration of the limewater.
For example:
if the mean volume of 0.02 mol dm-3 used was 30cm3
0.02 × 30 = 0.0006 moles of HCl
1000
HCl : Ca(OH)2
2 : 1
0.0006 : 0.0003
so there were 0.0003 moles of Ca(OH)2 in 25cm3
0.0003 = 0.012 mol dm-3
25/1000
0.012 × 74.1 = 0.88892g
so in this case the concentration of the calcium hydroxide solution would be 0.012 mol dm-3 or 0.88892g dm-3
Measurement Error Calculations
50cm3 burette 0.2 × 100 = 0.67%
(accurate to ±0.1cm3) e.g: 30
measured twice so 0.1 + 0.1 = 0.2
25cm3 pipette 0.06 × 100 = 0.24 %
(accurate to ±0.06cm3) 25
sum of measurement errors: 0.67 + 0.24 = 0.91
Hazards
Hydrochloric Acid –
As I am using this in a very low dilution then it will have irritant rather than corrosive properties, so this in itself greatly reduces the risk. Fumes may be harmful but usually only in stronger concentrations of acid, but to ensure safety I will perform the experiment in a well ventilated area. It is poisonous if ingested and can irritate skin and eyes. If swallowed, large quantities of water must be drunk but vomiting must not be induced, and wash hands after handling acid to reduce risk of ingestion. If it comes into contact with skin or eyes then wash thoroughly with water; so I will need to keep a supply of clean water close by. Also, to protect eyes and skin I will wear a lab coat and goggles. It is not considered to be a fire or explosion hazard nor is it very reactive.
()
Calcium Hydroxide –
Again the concentration I am using is very low but I will still follow most of the recommended safety practices. Avoid lost term exposure by inhalation by performing experiment in a well ventilated area. If ingested drink two glasses of water then induce vomiting. Has mild irritant properties, so long term contact with skin and eyes should be avoided; I will wear goggles and a lab coat to reduce the chance of any contact being made; but if some is spilt on my skin I will wash the area thoroughly with soap and water; if any gets into my eyes I will rinse with clean water immediately for at least 15 minutes lifting the lids occasionally. Not flammable or reactive.
()
I will immediately clean up any spillages using water and a paper towel, and to try and reduce the likelihood of spills I will immediately replace lids onto any bottles of solution once I have finished using them
I will also be handing a lot of glass equipment, so I must ensure the burette is firmly held in place by its stand, and I will not leave the burette or pipette anywhere may they roll off and smash. If a breakage does occur I will clear it up immediately using a dustpan and brush and put the shards inside some newspaper before I place them in the bin.
Bibliography
Ted Lister and Janet Renshaw (2004) Essential AS Chemistry for OCR Cheltenham
Nelson Thornes Ltd pg 23
Science Stuff (2006) MSDS: Limewater (Calcium Hydroxide Sat.) 24/02/2008
J.T Baker (2005) MSDS: Hydrocholic Acid (Less Than 10%) 24/02/2008
