Ksp of Ca(Oh)2

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Introduction:

Calcium hydroxide is a soft white caustic powder used in making mortar, cements, calcium salts, paints, and petrochemicals. It is also used in saltwater aquaria to make up kalkwasser/limewater solutions for reef tanks, and is used as a pH regulating agent. Notice that calcium hydroxide is divalent and thus has twice the neutralizing power as molecules like NaOH that are monovalent.

A Calcium Hydroxide Molecule:

Calcium hydroxide is manufactured industrially by adding water to calcium oxide (quicklime) in a strongly exothermic reaction: CaO(s) + H2O(l) → Ca(OH)2(s)

Calcium hydroxide, Ca(OH)2, is an ionic solid that is only slightly soluble in water. 

Solid Calcium Hydroxide:

A calcium hydroxide solution is also referred to as limewater. A saturated solution of calcium hydroxide has the solid in equilibrium with its ions as shown below:

Ca(OH)2(s) ↔ Ca2+(aq)  +  2OH-(aq)

Recall that a saturated solution is a solution that contains the maximum amount of dissolved solute possible at a given temperature. (The solution contains undissolved solute in equilibrium with the solution.)

Since calcium hydroxide is only slightly soluble in water, it is a difficult base to classify. It is often assumed that since calcium hydroxide has a low solubility that it is a weak base. But, don’t forget that it contains hydroxides ions, which automatically makes it a strong base! In fact, the pH of a saturated calcium hydroxide solution is about 12.4. Thus we can classify a saturated solution of calcium hydroxide as a dilute solution of a strong base.

A molecular view of a saturated solution of calcium hydroxide would look similar to the following, with clumps of undissolved calcium hydroxide at the bottom of the beaker in equilibrium with dissolved Ca2+ and OH- ions (although their would be twice as many OH- ions than Ca2+ ions):

[pic]

Note that the rate of dissolving is equal to the rate of precipitation in a saturated solution equilibrium.

In this type of equilibrium, the equilibrium constant is called the solubility product, and is represented by the symbol Ksp.  Whenever you see the symbol Ksp it refers to a solubility equation, written with the solid to the left of the equilibrium sign, and the dissolved products to the right. The Ksp for this reaction will be:

Ksp = [Ca2+][OH-]2

(The solid state is not included in a Ksp expression since it is a pure substance and cannot be expressed as a concentration).

Note that substances that have a large Ksp value have a higher solubility (more dissolved ions); and substances with a small Ksp value have a lower solubility (few dissolved ions).

Every substance that forms a saturated solution will have a Ksp.   However, for very soluble substances like NaCl, the value is so large that the concept is rarely used.  In low solubility substances, the value of Ksp is a useful quantity that lets us predict and calculate solubilities of substances in solution.  In this experiment you will collect the data that allows you to calculate the solubility of Ca(OH)2, and its Ksp value.

The concentration of hydroxide ions formed when Ca(OH)2 dissolves can be measured 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 color.  In the procedure used in this lab, a dilute solution of HCl is titrated with a saturated solution of Ca(OH)2 to the endpoint of phenolphthalein.

A saturated solution of calcium hydroxide must be made fresh on the day it is to be used as any carbon dioxide that enters the solution will cause it to react to form a calcium carbonate precipitate (chalk), as shown in the first two pictures below:

The equation is: Ca(OH)2(s) + CO2(g) ( CaCO3(s) + H2O(l)

Interestingly though, the precipitate will redissolve if...
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