Mol dm-3 = g dm-3/ Mr
So Mol dm-3 = 1 g dm-3 / 74.1
= 0.013495276653171390013495276653171
We’ll round this to 0.01 Mol dm-3 to make the dilution calculation easier. Because there is a 1:2 molar ratio we need to double the concentration of the Calcium Hydroxide to find the concentration of Hydrochloric Acid needed.
So:
Concentration of HCl = 0.01 x 2 = 0.02 Mol dm-3
The concentration of HCl we are given is 2.00 Mol dm-3. So now we need to find out by how much the acid is too concentrated.
2.00 /0.02 = 100 times too concentrated
So the acid needs to be diluted 100 times before using it in the titration. To dilute the acid this much we need to put 1cm3 of acid in a 100cm3 solution. The solution would then be 0.02 Mol dm-3.
To make this solution use a 2cm3 graduated pipette to place 1cm3 of the acid into a 100cm3 volumetric flask. Then fill the flask with distilled water to the mark. Then shake to make sure the acid is evenly dissolved. The solution is then ready for use in the titration.
We need to dilute the acid so that we get a good end point. A good end point is one that doesn’t use all of the solution in the burette and one that doesn’t use less than about 10cm3 of the solution in the burette. Therefore we are looking for an end point that is roughly in the middle. If the end point is unclear we will be unable to calculate an accurate concentration of the Calcium Hydroxide needed.
For this titration if we place the acid in the burette and the alkali in the conical flask there will be 25cm3 of alkali meaning we will need 12.5cm3 of acid. However if we place the acid in the conical flask and the alkali in the burette we will need 50cm3 of alkali to neutralise the acid which is the same amount of solution that a burette can hold.
Therefore we will need to use a 10cm3 bulb pipette to place the Hydrochloric acid in the conical flask so that we should need to use roughly 20cm3 of Calcium Hydroxide.
Final Apparatus
80cm3 Calcium Hydroxide, 40cm3 Hydrochloric Acid, Burette, Burette Stand, 15cm3 bulb Pipette, Pipette filler, Funnel, Conical Flask, 250ml beaker, Wash Bottle (distilled water), White tile, Methyl Orange Indicator, Safety Goggles
Method
- Rinse all apparatus with distilled water. Then rinse the burette with Calcium Hydroxide and the pipette with Hydrochloric acid. This is to make sure the distilled water doesn’t affect the concentration of the solutions and therefore make the experiment inaccurate.
-
Using the funnel, fill the burette with Calcium Hydroxide up to the 0 mark (make sure the tap on the burette is closed first). The bottom of the meniscus, of the solution, should be level with the mark. This should be checked at eye level, giving you an initial volume of 0cm3.
- Using the pipette filler, fill the pipette with Hydrochloric acid (which you have diluted in the volumetric flask) to the mark, checking the meniscus again at eye level. Then transfer the acid into a conical flask.
- Add a couple of drops of the Methyl Orange indicator into the conical flask with the acid and place the flask under the burette on a white tile. We are using Methyl Orange because the titration is using a strong acid and a strong alkali so this will be the best indicator as it will give a good colour change.
- Start running the Calcium Hydroxide from the burette into the conical flask while swirling and wait for a colour change. As it takes longer for the colour to disappear start adding the Calcium Hydroxide more slowly. However this is only a rough titration so it doesn’t matter if slightly too much Calcium Hydroxide is added as this is only an appropriate value.
- Record the volume of Calcium Hydroxide used (final volume) in a table of results.
- Repeat the titration more accurately by adding the Calcium Hydroxide drop by drop as the end point is approached. The end point should be roughly around the same as that of the first titration. This set of results should be accurate to 2d.p.
-
Do this at least twice or until concordant results are found (±0.1cm3).
