The safety precautions required for this experiment is to wear goggles to protect your eyes. The Hydrochloric acid at 2.00mol dm-3 can be an irritant to the skin, but not so much at 0.02mol dm-3.
I have calculated that in order to make up 250cm3 of 0.02mol dm-3 Hydrochloric acid. I will require 2.5cm3 of 2.00 mol dm-3 Hydrochloric acid made up to 250cm3 with distilled water.
The method I will use to dilute this Hydrochloric acid will involve me measuring out 2.5cm3 of 2.00 mol dm-3 Hydrochloric acid. This will be done using a 10cm3 graduated pipette, which gives the best possible accurate reading. I will add this 2.5cm3 of Hydrochloric acid to a 250cm3 volumetric flask. When adding the Hydrochloric acid from the pipette into the volumetric flask I will need to tap the end of the pipette to ensure all remaining Hydrochloric acid is removed from the pipette. Because I used a pipette to measure out the Hydrochloric acid, I will need to wash it out with distilled water. I am going to collect these washing because there may be some Hydrochloric acid traces in the pipette. Collecting the washings off all the equipment used will ensure there are no Hydrochloric acid traces left, which could affect my results. These will be added to the volumetric flask, and continued to be filled with distilled water up to the 250cm3 line. To ensure all the distilled water being added is kept in the body of the volumetric flask, not around its mouth, where it will not be measured, I am going to pour the distilled water down a glass rod through a funnel. To ensure I accurately fill the volumetric flask to 250cm3 I will maintain the 250cm3 line at my eye level, so it is accurate to see when the bottom of the meniscus is laying level on the line. To ensure the concentration of Hydrochloric acid is maintained throughout the solution, I will invert the flask 2-3 times, mixing the hydrochloric acid and distilled water together.
Method to Titrate hydrochloric acid and Limewater
2HCl(aq) + Ca(OH)2(aq) → CaCl2(aq) + 2H2O
The safety precautions required for this experiment to be safe, is that I will require goggles to wear. The concentration of solution I am using is not that high, therefore there are not many dangers in using these solutions.
I am going to find out the amount of 0.02 mol dm-3 Hydrochloric acid required to titrate 25cm3 of limewater. In order to do this I will accurately pipette 25cm3 of limewater. I will be provided with limewater in a large beaker, from this I can use a pipette to remove 25cm3, when drawing the limewater out I may overflow the 25cm3 mark. To remove a slight amount from the pipette, I will place my thumb over the top of the pipette and gentle wiggle it, this will let a minimal amount of air enter the pipette so that a slight amount of limewater is removed. To ensure the bottom the meniscus is level with the 25cm3 mark, I will keep the mark at my eye level. My thumb will tightly be kept over the top of the pipette when the pipette filler is removed. I will pipette the limewater 5 times, into separate conical flasks. I will stir the limewater solution before I take each 25cm3 aliquot to try to ensure as much calcium hydroxide has dissolved as possible. To each aliquot I am going to add 2 –3 drops of my chosen indicator, which is phenolphthalein. I will prepare the burette to fill. It will be filled with 0.02mol dm-3 Hydrochloric acid remembering to fill the portion below the tap. An accurate recording of the volume will be taken.
Underneath the tap of the burette I am going to place the conical flask with a white tile under, so that it is easier to see a colour change, when the Hydrochloric acid is being added. While constantly swirling the conical flask I am slowly going to add the Hydrochloric acid from the burette. (Because I am using a phenolphthalein indicator, which has a clear colour change from colourless to red when the required amount of Hydrochloric acid is added to neutralise the limewater, I will easily be able to tell when the required amount of Hydrochloric acid is added). At the end point the tap on the burette must immediately be closed so there is no excess of Hydrochloric acid added. I will record the amount titrated accurately to 0.05cm3. Once this experiment is complete I am going to repeat with the other 4 conical flasks. This will make my results more accurate as mistakes with adding too much or not enough Hydrochloric acid could easily be made, as the end point can be difficult to tell.
Suitable Indicator
When we add Hydrochloric acid to the limewater I will need an indicator to help me indicate when the required amount of Hydrochloric acid is added to achieve the end point, at which the titration is stopped. The indicators that be used:
We want an indicator with a clear, sharp colour change, this is because in many of these indicators the colour change can be unclear and the end point is hard to tell because there is a graduate colour change. Or the change colour is unlike because it is too similar to that before the end point.
I have chosen phenolphthalein as the best indicator, which I am going to use. This is because it is unlike the other indicators as it has a sharp colour change from colourless- red. There isn’t a graduate colour change making the end point hard to notice.
Hazards
Hydrochloric Acid
The solution of HCl we start with, is of 2.00mol dm-3, the hazards associated with HCl of this high concentration include:
After diluting the HCl it will be of 0.02mol dm-3, the hazards for this lower concentration include:
- Sensitive towards the skin.
Your eyes will need to be protected due to the dangers of splashing any of the solution, to do this goggles need to be worn.
Calcium Hydroxide
There are minimal hazards associated with this solvent, but it is advisable to protect the eyes by wearing goggles.
Calculations
I am provided with 250cm3 1gdm-3 Ca(OH)2
This means I have 1g Ca(OH)2 in 1000cm-3 water
We only want 250cm3 not 1000cm3
1000
250 = 4
Grams of Ca(OH)2= 1.00
4 = 0.25g
We are going to use 25cm3 pipette of Ca(OH)2
Grams in 25cm3 = 0.025g
Ca(OH)2 + 2HCl CaCl2 + 2H2 O
1 mole Ca(OH) 2 = mass = 0.025
Mr 74.1
= 0.000337moles
Got 0.000337 moles Ca(OH)2 in 25cm3
1 Ca(OH)2 : 2 HCl
0.000337 : 0.000674 moles
I am using 2.00 mol dm-3 HCl
Volume = moles x 1000
Concn
Volume = 0.000674 x 1000 = 0.337cm3
2.00
0.337cm3 : This volume of HCl is too small. I will be using a 50cm3 pipette, the accuracy will be too difficult to obtain. A reasonable volume will be between 20-35 cm3. In order to achieve a volume between 20-35 cm3 I will decrease the concentration.
If 0.02mol dm –3 HCl is used:
Volume = 0.000674 x 1000 = 33.7 cm3
0.02
This is a reasonable amount.
Dilute HCl from 2.00 moldm-3 0.02 mol dm–3
In a 250 cm3 volumetric flask.
2 ÷ 0.02 = 100
This means 0.02 mol dm-3 HCl is 100 times weaker than 2.00 mol dm-3.
250cm3 ÷ 100 = 2.5cm3
Therefore I need 2.5 cm3 2.00 mol dm-3 HCl and dilute it to 250 cm3 with distilled water.
A 10cm3 graduated burette will need to be used to get an accurate 2.5cm3 reading.
Bibliography
