Determining the Ksp of Calcium Hydroxide by Titration of Saturated Ca(Oh)2(Aq) with Hcl(Aq)

Topics: PH, Titration, PH indicator Pages: 7 (2028 words) Published: February 14, 2011
Determining the Ksp of Calcium Hydroxide by Titration of
Saturated Ca(OH)2(aq) with HCl(aq)

Titration is a technique that has been used in this experiment to identify the Ksp value of calcium hydroxide in order to determine the extent to which the compound is soluble in water. A known volume of 50 mL of hydrochloric acid, a concentration of 0.05 M hydrochloric acid, a volume of 50 mL calcium hydroxide base, an unknown concentration of base and an acid-base indicator (bromothymol blue) has been used in this experiment to determine the concentration of calcium hydroxide. From the results tabulated, a Ksp value of 1.26 x 10-7 has been retrieved indicating that calcium hydroxide has a low solubility in water. On the other hand, a value of 98.4 % error has been tabulated which indicates that there are both human and experimental errors which may have impacted the overall results.

Ksp is defined as the ratio of concentrations of dissolved ions and undissolved ions of a precipitate at equilibrium. This can also be expressed using the rate law equation. For example , the solubility product of calcium hydroxide can be written as, Ksp = [Ca2+] [OH-]2 (1)

This chemical system is in a dynamic equilibrium between solute and solvent particles in a saturated solution in a closed system. Therefore, the system is in solubility equilibrium. The solid, Ca(OH)2 dissolves in solution and the ions crystallize, therefore, the forward rate (dissolution) equals the reverse rate (crystallization). Since both dissolving and crystallizing processes take place at the same rate, no observable changes would occur in either the concentration of the ions in solution or in the quantity of solid present, Ca(OH)2. The mixture contains both dissolved and undissolved solute at the same time. At solubility equilibrium, the rate of the forward reaction equals the rate of the reverse reaction. As a result, until the rate of the forward reaction equals the rate of the reverse reaction, then the solution is saturated.

The concentration of hydroxide ions formed when calcium hydroxide dissolves in solution can be determined using the titration technique. 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 colour. In the procedure performed in this lab, a solution of HCl is titrated with a saturated solution of Ca(OH)2 to the endpoint of bromothymol blue. Bromothymol blue is a good indicator because it changes color from yellow to blue within a pH range of 6.2 to 7.6. The indicator changes color when the equivalence point is also reached in the above mentioned pH range. The equivalence point is the point in which the concentration of hydroxide ions is equal to the concentration of hydronium ions. This is when equilibrium is reached between the calcium hydroxide and the hydrochloric acid. The equivalence point can be written as a pH value on the pH scale, in this titration between a strong acid and a strong base the equivalence point is around a pH of 7 (neutral point). Therefore, bromothymol blue is used in this lab because the equivalence point falls inside the range where the indicator changes color.

Strong Acid + Strong Base
HCl(aq) + Ca(OH)2(aq) Produces a solution with a pH of 7.00 (neutral)

The purpose of this experiment is to determine experimentally the solubility product of aqueous calcium hydroxide by titrating it with a solution of hydrochloric acid with a known molarity until equilibrium is reached; this is performed using the titration technique. The hypothesis of this experiment is that the experimentally determined Ksp will be...
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