0.0375 / 74 = 0.0005 moles
Ca(OH)2(aq) + 2HCl → CaCl2(aq) + 2H20(l)
As seen from the equation the ratio of hydrochloric acid to calcium hydroxide is 2:1, therefore we need twice as much hydrochloric acid. The number of moles of hydrochloric acid required to react with 0.0005 moles of calcium hydroxide will be:
0.0005 x 2 = 0.001 moles
Ideally we will want the same volume of hydrochloric acid reacting with calcium hydroxide, which is 25cm3 and the concentration would therefore be:
Moles = 0.001 = 0.04 mol/dm3
Volume (25/1000)
Method
Accurately weigh out 0.875g of calcium hydroxide using a weighing boat and electronic scales. I will firstly weigh the weighing boat, then zero the scales, and then add accurately and carefully, 0.875g of calcium hydroxide. Then get the funnel and place it in the conical flask. Then carefully pour the calcium hydroxide into the funnel, so it goes into the volumetric flask, as shown in the diagram below.
As always, there will be some calcium hydroxide in the funnel, and to ensure it all goes in the volumetric flask, pour distilled water into the funnel, so all the calcium hydroxide gets washed into the volumetric flask. Then you need to make it up to the bottom of the meniscus, which will be 250cm3, by adding distilled water as shown in the diagram below. Make sure that when filling it up the line, that you do it very slowly, as if you go over, you cannot pour it out, as then you will lose some of the calcium hydroxide. Once filled to the line, put a rubber bung on the flask, and mix the solution. Then allow it to settle down, so that the excess calcium hydroxide that was added, may be able to go down to the bottom, as shown in the diagram below.
Then attach the pipette to the pipette filler and use the calcium hydroxide solution in the volumetric flask to rinse out the pipette. After being rinsed, take out 25cm3 of calcium hydroxide from the volumetric flask, using the pipette. 25cm3 is easily measured on the pipette, as there is a mark line. Be careful when doing this, as if you suck up the solution too quickly with the pipette, then air gaps form in the pipette, and this means you will not have a full 25cm3 of solution, making the experiment unreliable.
After, release the solution of calcium hydroxide into the conical flask. Add four drops of methyl orange indicator to the conical flask, and the solution will now turn orange.
Titration
The burette will be rinsed out with 0.04 mol/dm3 hydrochloric acid, so during the experiment, the hydrochloric is not contaminated with solutions previously used in the burette. Before rising out the burette, make sure that the tap is closed in the horizontal position. Bring the burette down to a level where you can be pouring into it at your height level, so you do not have to reach up, otherwise this could be a hazard. Place a funnel inside the top of the burette, and carefully pour the hydrochloric acid into the burette via the funnel. Once full to the top, open the tap, and let the dirty hydrochloric drain into a beaker. Once the burette has been thoroughly rinsed, close the tap again, and pour in fresh 0.04 mol/dm3 of hydrochloric acid. Fill the burette to the top, where the reading will be 0cm3. Ensure the bottom of the burette below the tap is full of hydrochloric acid, so all the readings are accurate.
Place a white spotting tile under the tip of the burette, and then place the conical flask containing the solution onto the spotting tile. The spotting tile is there to help establish when the neutralisation between the acid (hydrochloric acid) and alkali (calcium hydroxide) has fully occurred. The equipment will be set up as below:
Before beginning any one of the titrations, always record the initial volume from the burette. Once the initial volume is recorded, open the tap of the burette and allow the acid to pass into the conical flask slowly. Make sure it is done slowly otherwise the neutralisation may happen to quickly before you can close the tap. While the acid is being added to the conical flask, swirl the conical flask, and when the colour begins to change, only let the hydrochloric acid out of the burette drop by drop, to ensure most accurate results.
When the neutralisation has finished, the solution will go from orange to a red/pink colour. This is why we have a spotting tile, to ensure that the same colour is reached in every titration before we close the tap, and measure of the reading from the burette. Now it is possible to work out the amount of acid used, by taking away the final volume in the burette, from the initial volume in the burette.
As I am using a 250cm3 volumetric flask, I will be able to carry out a maximum of 9 titrations, as one lot of 25cm3 was used to rinse the pipette. However, I will repeat the experiment until two or three results are within the range of +/- 0.2cm3, and then find the mean of the volumes.
After one titration depending on how much hydrochloric acid is left in the burette, fill it up again, so there is a full 50cm3 in it. Again follow all the safety tips when filling the burette up. You have to also rinse the conical flask out with distilled water, before adding another 25cm3 of calcium hydroxide solution. Then follow the method used for the first titration.
The results will be recorded in a table which has rows, with the initial volume reading of the burette, final volume on burette, and the volume of acid used. The columns will be the titration number, and the end column will be the mean volume used.
Calculation of the solubility of calcium hydroxide
- Moles of Hydrochloric acid:
= concentration x volume
= 0.04 mol/dm3 x mean volume
- There is for every one moles of calcium hydroxide, two moles of hydrochloric acid, therefore we need to divide the amount of moles of hydrochloric acid by two, to get the number of moles of calcium hydroxide.
The number of moles of Ca(OH)2 = mean x 0.04
in one solution of volume 25cm3 2
-
We want the number of moles of calcium hydroxide in 1dm3 of water, therefore we need to work out its concentration.
Moles of Ca(OH)2 in dm3 = mean (dm3) x 0.04 x 1000
2 x 25
-
The molar mass of calcium hydroxide is 74. If we multiply the concentration of calcium hydroxide by the molar mass, then this will tell us the amount of solid calcium hydroxide which will dissolve in 1dm3 of water.
20
Solubility of Ca(OH)2 in g/dm3 = mean x 0.04 x 1000 x 74
2 x 25
= mean x 0.04 x 20 x 24
= mean x 59.2 g/dm3
