Chemicals used are:
- 250cm3 of limewater which contains approximately 1g dm-3of CaOH
- HCL solution (2.00 mol dm-3)
- Distilled water
- Methyl orange used as an indicator
Firstly put the alkaline calcium hydroxide solution in the flask and the hydrochloric acid in the burette.
Next you need to construct the relevant equation:
Ca(OH)2 + 2HCl ----> CaCl2 + 2H2O
The above equation tells us that 1 mole of calcium hydroxide reacts with exactly 2 moles of hydrochloric acid, so we should try to make the solution of HCl about twice as concentrated as the Ca(OH)2, so that when we measure out, shall we say, 10 cm3 of the calcium hydroxide solution into the flask we should need to add approximately 10 cm3 of hydrochloric acid from the burette. You have already been told that the acid with which you have been supplied is too concentrated in its present form, so if it was used directly in a titration a ridiculously small volume would need to be added, maybe as little as 0.1 cm3 (which would be far too difficult to measure accurately).
The quantities and concentration
I was told that the Ca(OH)2 solution contains about 1gm-3
Molar mass of CaOH =
( Ca=40, O=16, H=1)
∴moar mass CaOH=40+ (1×2+16×2) = 74g
the concentration of the solution = 1
74
= 0.0135 mol dm-3
As I said above I should try to make the concentration of the acid about twice as much as this, i.e. 0.0135 × 2 = 0.027 mol dm-3.
the acid’s present concentration is 2.00 mol dm-3 (far too high), so it must be diluted 2.00/0.027 times, i.e. about 70 times. The most accurate way of doing this is to fill a burette with the acid of concentration 2.00 mol dm-3, and run exactly 3.4 cm3 of this into a 250 cm3 volumetric flask.
Next add distilled water and make up “to the mark.
Ensure then that the flask is upended several times to ensure thorough mixing.
This dilution of 3.4 cm3 into 250 cm3 will give us an acid which has a new concentration of:
(3.4/250) x 2.00 = 0.027 mol dm-3.
Next I need to use a pipette to transfer 10.0 cm3 of the calcium hydroxide into a conical flask, add 3drops of a suitable indicator (methyl orange)
then I will carry out the titration with the diluted acid.
You should find that you obtain a titration reading of about 10 cm3, but you will need to work accurately and obtain at least three consistent readings.
The final step is of course to calculate the precise concentration of the calcium hydroxide solution
