Write the balanced equation and the ksp expression for the dissolving of calcium

Question

Write the balanced equation and the ksp expression for the dissolving of calcium hydroxide and magnesium hydroxide

***Update***

1. What is the value of Ksp for MgOH2?

2. What is the concentration of the OH- in a saturated solution of MgOH?

3. What is the molar solubility of MgOH?

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Obtain about 100 mL of a saturated calcium hydroxide solution.

Caution:

The calcium hydroxide solution is caustic.

Handle with care. Clean up any spills immediately.

Set up a ring stand, ring, filter funnel, and filter paper. Filter your sample of Ca(OH) 2   solution into a clean beaker. Using a pipet, transfer 25.00 mL of the filtered solution into a 250 mL beaker.

Obtain about 200 mL of 0.050 M HCl solution.

Caution:

Handle the hydrochloric acid with care. It can cause painful

burns if it comes in contact with the skin.

Connect a buret to the ring stand. Rinse and fill the buret with the 0.050 M HCl solution.

Set up the beaker of Ca(OH) 2   solution on a magnetic stirrer. If you are not using a magnetic stirrer, use a glass stirring rod to stir the solution throughout the titration.

Make sure that the magnetic stirring bar does NOT come in contact with the tip of the pH meter. Conduct the titration by adding enough HCl so that the pH changes by about 0.15 units per addition. This may required a few mL of HCl at the start of the titration and only a drop or so near the equivalence point. Add enough HCl past the equivalence point so that the pH flattens out to make sure you have properly identified the end point of the titration. Accurately record the volume and pH for each point.

Dispose of the reaction mixture as directed. Rinse the pH meter with distilled water in preparation for a second titration.

Examine your titration data to identify the region where the pH made the greatest decrease. The equivalence point is in this region. A derivative plot is useful in determining the pH at the equivalence point.

Repeat the necessary steps to titrate a second, and/or third, sample of the filtered Ca(OH) 2   solution. Every member of the group should do at least one titration, and you should perform at least three (3) titrations.

Clean up your work area(s), properly dispose of all waste, and return all equipment to its proper location.

Determine the average [Ca 2+ ] , average [OH ? ] , and average K sp   for your titrations and record them on the board. Make sure to record the results from all the other students in you lab notebook as you will need these when you calculate the population standard deviation.

Caution:

The calcium hydroxide solution is caustic.

Handle with care. Clean up any spills immediately.

Explanation / Answer

Mg(OH)2 (s) <----> Mg+2 (aq) +   2OH- (aq)

S mol/L                       S mol/L    2S mol/L

Ksp = [Mg+2] * [OH-]^2

a) Ksp for Mg(OH)2 = 1.8 *10^-11

let molar solublityof Mg(OH)2 be S mol /L

Ksp = [S] * [2S]^2 = [S] * 4S^2 = 4S^3

1.8 *10^-11 = 4S^3

1.8 *10^-11 /4 = S ^3

0.45 *10^-11 = S ^3

S=1.65 *10^-4 mol/L

Molar solublity of Mg(OH)2 = S = 1.65 *10^-4 mol/L (answer)

concentration of the OH- = 2S = 2 * 1.65 *10^-4 =3.3 *10^-4 mol/L (answer)

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