Molar Solubility

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  • Topic: Solubility equilibrium, Calcium hydroxide, Solubility
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  • Published : April 11, 2013
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Mateo Castro
April 3, 2013
Lab Partner: Unur Abdul Kader
T.A: Katie

Experiment 22: Molar Solubility, Common-Ion Effect

Abstract
The purpose of this experiment was to determine the molar solubility, the solubility constant, and the effect of a common ion on the molar solubility of calcium hydroxide. To accomplish this the experiment was split into two parts; part A and Part B. in Part A of the experiment a standardized 0.05 M solution of HCl was titrated into a 25 mL solution of saturated Ca(OH)2 which contained 2 drops of orange methyl identifier. Once the titration began, the HCl was added until the methyl orange endpoint was reached, and as a result the volume of the HCl needed for the endpoint to be reached could be used in determining the moles and in turn the molar solubility and the solubility constant of the Ca(OH)2. For the second part of the experiment a common ion (CaCl2) was added to the calcium hydroxide and the process was repeated to analyze how the common ion would affect the molar solubility of the calcium hydroxide solution.

The purpose of this experiment was to resolve for calcium hydroxide’s molar solubility, its solubility constant, and to observe the effect that a common ion will have on the molar solubility of the calcium hydroxide. In this experiment calcium hydroxide is a salt that is slightly soluble, meaning that the salt has a very limited solubility in water (Beran, 2011). The lack of solubility of the salt can be observed by the saturated solution that is created at the dynamic equilibrium of the salt and the water, however since the salt is only slightly soluble, the concentration of the ions in the solution is low and the dynamic equilibrium is located to the left of the reaction or on the side of the reactant. In this experiment a titration using a standardized HCl solution will be used to determine the solubility product of the dissolution of calcium hydroxide in water. The following chemical equation can explain the dissolution of the calcium hydroxide in water: Ca (OH)2 (s) Ca2+ (aq) + 2 OH- (aq)

It is clear to see that the concentration of calcium is half of the concentration of hydroxide. The molar solubility of this reaction can be determined with the use of the chemical equation and some relations determined from the same chemical equation (Beran, 2011). The ksp of the reaction is determined to be equal to [Ca2+][OH-]2= ½[OH-]3, and the molar solubility of calcium hydroxide can be determined with the relationship between the [OH-] and the [Ca2+], which would equal: the molar solubility of Ca(OH)2= [Ca2+]=1/2[OH-].

Experimental
The materials and the methods used in this experiment can be acquired from J.A. Beran’s Laboratory Manual for Principles of General Chemistry; Ninth Edition. Experiment 22: Molar Solubility, Common-Ion Effect. For this experiment there was only one exception. The number of trials for part one and two of the experiments were decreased to two trials instead of three. Results

Data
Table 1: Molar Solubility and Solubility Product of Calcium Hydroxide. | Trial 1| Trial 2|
Volume of saturated Ca(OH)2 solution (mL)| 25| 25|
Concentration of standardized HCl solution (mol/L)| 0.05| Buret reading, initial (mL)| 0| 0|
Buret reading, final (mL)| 25.8| 25|
Volume of HCl added (mL)| 25.8| 25|
Moles of HCl Added (mol)| 0.00129| 0.00125|
Moles of OH- in saturated solution (mol)| 0.00129| 0.00125| [OH-], equilibrium (mol/L)| 0.0516| 0.05|
[Ca2+], equilibrium (mol/L)| 0.0258| 0.025|
Molar solubility of Ca(OH)2 (mol/L)| 0.0258| 0.025|
Average molar solubility of Ca(OH)2 (mol/L)| 0.0254|
Ksp of Ca(OH)2| 6.87E-05| 6.25E-05|
Average Ksp| 6.56E-05|
Standard Deviation of Ksp| 4.38E-06|
Relative standard deviation of Ksp| 3.10E-06|

Table 2: Molar Solubility of Calcium Hydroxide in the Presence of a Common Ion. Volume of saturated Ca(OH)2 with added CaCl2 solution (mL)| 25| 25|...
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