Nathalie Dagmang

Co-worker: Annjaneth Briones

Group 8

Date Performed: January 11

Results and Discussion Report 6:

Determination of the Solubility Product Constant of Calcium Hydroxide

Some systematic studies are made that showed that at some point, the K sp of a slightly soluble ionic solid depends on the ionic strength rather than the chemical nature of its ions. The quantity of ionic strength is defined as:

μ=12∑C I Z I

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where Ci is the concentration of each ion while the Z i is the respective charge of the ion. The calculated quantity for ionic strength is only used for solutions with 0.1M ionic strength or less. If the value exceeds 0.1, the solubility now depends on the nature of the ions.

This effect is a result of the attractive and repulsive forces that exist between the ions of the solid and of the medium. Because the ions of the solid are slightly charged, the ions of the added solution will surround each of the solid’s ions. In effect, the solid’s ions are blocked from each other, decreasing the attraction between them and increasing the extent to which they will be separated. Hence, the solid’s ions will be less effective and its solubility will be higher when dissolved in a medium with a high ionic strength.

The reaction investigated in this experiment is the precipitation reaction described as:

Ca2+(aq) + 2OH- Ca(OH)2(S)

for which the solubility product constant (Ksp) expression is:

Ksp=¿

Because in the laboratory, there are no means in determining the amount of each ion that was dissolved, a titration method was used after the suspension of the precipitate in different medium. This was done in order to determine the amount of hydroxide ions, hence its solubility and the solubility of Ca2+ ions which is only equal to half the amount of hydroxide ions. Therefore, the solubility product constant was calculated using the equation:

Ksp=¿¿¿

Ksp=¿¿¿¿

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Using this method, it was found out that the solubility of calcium hydroxide precipitate decreased when dissolved in a medium containing a common ion, in this case, calcium ion from the calcium nitrate solution. This “common-ion effect” can be explained by the Le Chatelier’s principle. This principle states that when a stress is added to a reaction, the solution reacts such that it reduces the stress until it reaches equilibrium. In accordance to this principle, when a common ion is added to the reaction:

Ca(OH)2(S) Ca2+(aq) + 2OH-

the dissolution process precedes to the opposite direction so as to consume the added ion, increasing the extent to which the solution forms a precipitate.

It was also investigated in this experiment the effect of ionic strength in the solubility of calcium hydroxide. This was done by suspending the precipitate in different concentrations of KNO3 solutions and using the same method of determining the Ksp stated beforehand.

The results showed that as the concentration of the medium increases, the solubility of the precipitate also increases. This is because the number of moles present increases as the concentration increases, thus increasing the amount of ions that will surround the ions of the solid. This can be explained by the concept of the ionic strength and its effect on solubility which is explained above. In this case, the positively charged K+ ions surrounded the partially negative OH- ions while the NO3

- ions surrounded the partially positive Ca2+ ions. This process resulted to the decrease of attraction between Ca2+ and OH- ions, making these more separated from each other.

References:

Oulette, R. 1988.Introduction to General Chemistry and Biological Chemistry, Second Edition.

Skoog, West, et al. 1996. Fundamentals of Analytical Chemistry, 7th edition.

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