I am working on the discussion of a lab titled Ksp of Ca(OH)2. I found the Ksp t

Question

I am working on the discussion of a lab titled Ksp of Ca(OH)2. I found the Ksp to be 8.64x10-13.

Explain whether each of the errors below would cause your calculated Ksp value to be higher or lower than the tabulated value. Assume you are using method 1 (titration curve) to calculate Ksp. It is also possible that the error would have no effect on the measured Ksp value (so it really wouldn’t be an error).

1. The calcium hydroxide solution was prepared at the last minute. Adequate time was not given for equilibrium to be reached between the solid and dissolved calcium hydroxide. (These solutions are prepared by adding solid calcium hydroxide to DI water and stirring it for a long time.)

2. The burette was not rinsed with 10 mL of HCl and HCl was not run through the stopcock before it was filled with the acid and some DI water remained on the sides of the burette and in the stopcock.

3. The pH meter was not calibrated before the experiment. All the pH readings were 0.2 pH units too high.

4. The HCl was not diluted correctly. Although it was labeled 0.020 M, it was really 0.030 M.

Explanation / Answer

An acid-base titration is a process in which a measured volume of an acid or base is added to a reaction mixture until the acid-base indicator changes color.

In the procedure used in a lab dilute solution of HCl is titrated with a saturated solution of Ca (OH)2to the endpoint of phenolphthalein.

A saturated solution of calcium hydroxide is made fresh on the day because any carbon dioxide that enters the solution will cause it to react to form a calcium carbonate precipitate (chalk), as per the equation

Ca(OH)2(s) + CO2(g)   à CaCO3(s) + H2O(l).   

The equation and the practical value of a saturated aqueous solution of Ca (OH)2 is:-

  Ca (OH)2 (s) Ca2+ (aq) + 2OH– (aq) and solubility product of Calcium hydroxide Ca (OH)2 comes 5.5×10–6.

Further the factor which effect the Ksp are Temperature, Diverse-ion effect , Common-ion effect , Simultaneous equilibria etc.

Because

i. The given Calcium hydroxide solution was prepared at the last minute and adequate time was not given for equilibrium to be reached between the solid and dissolved calcium hydroxide.

ii. The burette was not rinsed with 10 mL of HCl and HCl was not run through the stopcock and properly not washed with distilled water.

iii. The pH meter was not calibrated before the experiment and all the pH readings were 0.2 pH units too high and the HCl was not diluted correctly,

the value of Ksp is = 8.64x10-13 .

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