- 250cm3 volumetric flask
- 5cm3 graduated pipette
- pipette filler
- white tile
- clamp and stand
- conical flask 250cm3
- 100cm3 beaker
Ca(OH)2 + 2HCl CaCl2 + 2H2O
Firstly I will calculate the approximate concentration of the calcium hydroxide in mol dm-3
Moldm-3 = gdm-3/Mr = 1/74.1 = 0.014 (3.d.p)
Now I will use the balanced equation to work out the approximate concentration of HCl needed. To work this I would need to know the volume ratio, and as the volume ratio is (1: 2) the acid will need to be twice as concentrated.
0.014 × 2 = 0.028 or (0.03 to 2.d.p)
To acid that was given was too concentrated so I need to find the dilution factor too show how concentrated it is.
Dilution factor = original concentration/concentration needed
Original concentration: 2.00
Concentration needed: 0.03
Dilution factor = 2.00/0.03
Dilution factor = 66.67
Dilution of HCl acid:
Before I start this experiment I will need to dilute the acid as it is too concentrated to be used. Various apparatus are needed to carry out this dilution and the equation that can be used to do this is:
Volumetric flask/pipette volume = 66.67
My chosen volumetric flask is 250cm3 so my pipette volume = 250/66.67 = 3.75cm3 therefore as this is just an approximate I will round this number to 4.00 cm3
This requires a 5 cm3 graduated pipette.
I will pipette 4.0 cm3 of 2.00 mol dm-3 acid from the 5.0 cm3 graduated pipette into a 250cm3 volumetric flask.
Subsequently make the solution so that it is up to the mark on the 250cm3 with distilled water and shake thoroughly to make sure the solution is of a consistent concentration.
The new concentration of this acid is:
4.0cm of 2.00 mol dm-3
Moles = concentration × volume
Moles = 2.00 × 0.004
Moles = 0.008
Concentration = moles/volume
Concentration = 0.008/0.25
Concentration = 0.032 mol dm-3
I will round this number up as it is just an approximation to 0.030 mol dm-3
As a result this solution has a new concentration of 0.030 mol dm-3
- Firstly collect I will all the chemicals needed in separate beakers and label them all.
- Then I will rinse the burette with distilled water and then with calcium hydroxide.
- After I have rinsed the burette I will fill it with the calcium hydroxide up to the zero mark, whilst I do that I will make sure that the bottom of the meniscus is on the zero mark and I also made sure that there were no air bubbles at the tap end and that they are all removed.
- I will then put some of my hydrochloric acid solution into a 100 cm3 beaker.
- Using the pipette filler I will remove some of the acid from this beaker and use it to rinse my 25cm3 bulb pipette.
- I will then fill my pipette to the 25cm3 mark making sure the bottom of the meniscus is exactly on 25cm3. I will then transfer this hydrochloric acid to a clean conical flask.
- Subsequently I will add three or four drops of methyl orange as that is the indicator that I have chosen to use. The colour of the solution will change from a colourless solution to a pink
- I will then open the tap on the burette and let the calcium hydroxide solution run in the conical flask which contains the hydrochloric acid solution until the solution changes from a pink to an orange colour.
- Whilst adding the calcium hydroxide I will swirl the conical flask constantly to see the colour change as I have to make sure that I don’t add too much of my alkali solution in and make sure that the colour change lasts for at least 30 seconds.
- I will record this result as my trial result as I may not get the right colour or I will use it to compare the colour with the rest of my results.
- After that I will rinse my conical flask thoroughly with tap water then with distilled water.
- I will then repeat step 8. This time when the volume is 2cm3 less than the volume required add the calcium hydroxide drop by drop so I don’t add too much. The main thing that I will note whilst doing this titration the colour of the solution must be the palest pink. This is the end point
- repeat step 12 at least twice more times until I get concordant results of 0.1cm3
- When I have done this I will rinse all my equipment out and put them away.
R:/dept/CHEM/OCR/OCR coursework/coursework/sodium carbonate.doc
Here's what a star student thought of this essay
Quality of writing
Punctuation, grammar and spelling all done to a good level. The candidate lays the essay out in a good format. The referencing could have been done a bit tidier by using the harvard referencing format.
Level of analysis
The candidate only explains weakly why they will use the methyl orange indicator. To improve they should compare other indicators and explain their choice with its properties for thoroughly for the suitability for the experiment to go ahead. They explain the safety elements behind the experiment well. The equation used is in a correct format with correct balancing used. The dilution and titration methods used are calculated accurately and explained well. Decimal places lack consistency in one or two places and this could affect the accuracy of the results.
Response to question
The focus of the question is done well. The candidate provides accurate worked out solutions and explains how to dilute the HCl to the desired concentration. The main thing missing is the ending of the experiment which is not included in this format.