Equilibrium law
Read our general notes on Risk Assessment
Hello, I've done a "Solubility product of calcium hydroxide and the common ion effect" lab where saturated solutions of calcium hydroxide were prepared in water and 0.10M sodium hydroxide and titrated against 0.05M hydrochloric acid. My titre values for water and NaOH are 9.83 and 24.08 cm3 respectively.
I calculated the Ksp of Ca(OH)2 in water to be 5.93 x 10-5 which is a whole decimal point away from the data book value. This worries me.
Further, we are to calculate Ksp in sodium hydroxide using of course the equilibrium concentrations of ions present but I am having major difficulty with this. Your pending advice is much appreciated.
Igloo writes ...
You made it difficult for me to check your calculations, since you forgot to tell me the capacity of the pipetted samples being used. I deduced that you must have used a 10 cm3 pipette, and yes, your solubility product is about 10 times higher than it should be.
Using a solution which was not saturated would have given a low answer, so clearly this wasn’t the problem. Assuming that you carried out the experiments carefully, and that the hydrochloric acid was in fact 0.05 molar, this only leaves one explanation for your result.
Solubilities can vary significantly with temperature, and, in the case of calcium hydroxide, the solubility and hence the solubility product happen to increase as the temperature decreases. This, of course, is not what you would expect, but is a feature of calcium hydroxide, calcium sulphate and a few other inorganic compounds. The quoted value of about 5x10-6 mol3 dm-6 applies to saturated solutions of calcium hydroxide at 25°C. Even if your solution was only a few degrees lower, say at 22°C, the solubility would be higher, and this would have a really significant effect on Ksp since one concentration is being squared before being multiplied by that of the other.
If you used a thermostat at 25°C, then I can only assume that your practical technique has been faulty in some way.
Let me have your comments on all this, and if you want I can then help you with the calculation for the Ksp in NaOH(aq), which, of course, should be the same as that for Ca(OH)2 in water.
Always carry out a risk assessment and check with your teacher before starting any practical work.
Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website.
For further guidance see our tutorial on Risk Assessment.
Rate this page or react
Share your views on this page, 0 ratings so far
updated: 01 May 2007
