Determination of the Solubility of Calcium Hydroxide

Determination of the Solubility of Calcium Hydroxide

Apparatus

* Solid calcium hydroxide

* 0.4 mol/dm hydrochloric acid

* Distilled water

* Pipette

* Triple valve rubber pipette filler

* Conical flask

* Beaker

* White tile

* Clamp and stand

* Methyl orange indicator

Producing the calcium hydroxide solution

1. Roughly fill a beaker with 200cm³ of distilled water. This does not need to be accurate because samples will be taken from this.

2. Add solid calcium hydroxide, a spatula at a time, and stir to aid the dissolving.

3. Continue to add the calcium hydroxide until no more will dissolve
i.e. you can see the solid at the bottom, even after much stirring. The amount of calcium hydroxide needed should be around the value of 0.3g.

4. Fold a piece of filter paper in half, and in half again and pull one side open to form a cone.

5. Place this cone inside the funnel.

6. Set up a container underneath the funnel.

7. Pour the saturated calcium hydroxide solution through the funnel and into the container beneath. All undissolved calcium hydroxide has now been removed.

At this point it is essential to calculate the molarity of the acid that will be used in the titration.

Setting up the titration equipment
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8.Set up the equipment for the titration showed in the diagram below

9. Wash out a conical flask with distilled water.

10. Wash out a 25cm pipette with the calcium hydroxide solution, by drawing up the solution with a pipette filler.

11.Take a 25cm sample of the calcium hydroxide solution by using a
25cm pipette and a triple valve rubber bulb pipette filler. (See the diagram below)

12.Squeeze the large bulb at the top of the pipette filler and then squeeze valve 1 to draw the liquid up the pipette. If the level of the liquid goes higher than that of the mark for 25cm then squeeze the 2nd valve to release the liquid

13.When the level of the liquid is exactly at the mark (i.e. the bottom of the meniscus lies on the line when looking at it at a 90 degree angle), then carefully and quickly remove the pipette filler, and put your index finger on the top open end of the pipette.

14. Place the pipette over the conical flask and by removing index finger from the open end of the pipette the calcium hydroxide solution will run into the conical flask. When nearly all of the solution has gone out of the pipette gently tap the pipette on the bottom of the conical flask to get another drop out. There is no need to worry about the drop that is still in the pipette because this was accounted for when the pipette was made.

15. To wash the burette out close the stopcock at the bottom and use a funnel. 16. Pour the 0.4 mol/dm hydrochloric acid into the funnel.

17. Drain the burette into a beaker by opening the stopcock.

18. Check the tip of the burette for an air bubble. To remove an air bubble, tap the side of the burette tip while solution is flowing.

19. Close the stopcock and fill the burette with the hydrochloric acid, using the funnel. It may help with calculations to fill the burette to 0cm. It does not matter if it overfills to just above the
0cm mark because the excess can be drained off into a beaker, by opening the stopcock.

20. Take an initial reading and write it down. Remember to take the reading from the bottom of the meniscus (see the diagram below), when your eyelevel is at the level of the meniscus and not above or below.

21. Place the conical flask containing the calcium hydroxide underneath the burette.

22. Put a few drops of methyl orange into the conical flask. The colour of the methyl orange now is a yellowy colour.

23. Place a white tile underneath the conical flask. This will aid in detecting the colour change.

Performing the titration

24.Open the stopcock and deliver a stream of the hydrochloric acid into the conical flask. Swirl the flask throughout.

25. When you see the colour change (it will turn to a reddy orange colour) then you know that the solution has been neutralised. Close the stopcock.

26. Record this reading. This will be the rough reading. Estimate how much it overran and remember this for the time you take the next reading. 27. Empty out the contents of the flask and wash it out with distilled water. Refill the flask with 25cm of the calcium hydroxide solution using the same process of filling it as before. Add a few drops of methyl orange as before.

28. Repeat the titration, this time slowing the stream of the hydrochloric acid when it approaches the expected endpoint. You will see the indicator change colour when the titrant hits the solution in the flask, but the colour change disappears upon stirring.

29. Approach the endpoint much more slowly and watch the colour of the solution of the flask carefully. As you approach the endpoint, you may need to add a partial drop of titrant. You can do this with a rapid spin of a Teflon stopcock. If you think you might have reached the endpoint, record the volume reading and then add another partial drop.
It is sometimes easier to tell when you have gone past the endpoint.

30. When you have reached the endpoint read the final endpoint and record it. Subtract the initial volume to determine the amount of hydrochloric acid used.

31. Repeat the titration using the steps above until you have 3 results for the amount of hydrochloric acid needed which are the same to 0.1 decimal points.

Calculating the solubility of calcium hydroxide

From these results it is possible to calculate the solubility of calcium hydroxide. First a mean average of the 3 results (the same to
0.1 decimal points) must be calculated. The 3 results must be added together and then divided by 3.

This result will be taken as the quantity of 0.04mol/dm hydrochloric acid required to neutralise 25cm of an unknown molarity of calcium hydroxide. The solubility of calcium hydroxide can then be calculated using the equation [IMAGE]Ca(OH) (aq) + 2HCl CaCl (aq) + 2H O

The amount of moles that were needed of hydrochloric acid needed to neutralise an unknown amount of moles of 25cm of calcium hydroxide can be calculated by

[IMAGE]Quantity of hydrochloric acid x 0.04

1000

Due to the ratio of 2 moles of hydrochloric acid to 1 mole of calcium hydroxide, the amount of moles of calcium hydroxide used in the equation can be calculated by divided the answer from above by 2.

Now we know the amount of moles in 25cm we can calculate the amount of moles of calcium hydroxide there would be in 1dm . This is done by multiplying the previous answer by 40 (1000/25).

It is necessary now to calculate the formula mass of Calcium hydroxide. Ca(OH)

40+(2 x 16) + ( 2 x 1) = 74g

This is the mass of one mole. To work out the mass of calcium hydroxide in 1dm we must take the answer from before (the amount of mole in 1dm) and then multiply this by the formula mass of 74g.

Thus the solubility of calcium hydroxide in units of g dm at room temperature has been found.