K_sp for Calcium Hydroxide

Introduction

In this experiment, you will use a measured volume of calcium hydroxide saturated solution and neutralize it with a solution of known concentration of hydrochloric acid. You will know when the neutralization is complete by the change in color of the indicator in the solution. From the volume of acid used, you will be able to determine the Ksp and the concentration of calcium hydroxide.

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1. Measure the temperature of the stock calcium hydroxide.
2. Select a Beral pipet to use for the reagents. Fill the pipet with distilled water and then squeeze dry. Fill the pipet with the stock calcium hydroxide.
3. Carefully transfer 50 drops of calcium hydroxide to each of 3 wells in a 24 well plate. Add 2 drops of phenolphthalein to each well.
4. Empty the left over calcium hydroxide into the sink and then wash with distilled water and squeeze dry. Fill again with distilled water and discard the water. (Instead, you may wish to use the two-piece device consisting of a plastic bulb and a pipet tip described above.)
5. Fill the Beral pipet with 0.1M HCl from the stock supply.
6. Add the acid to a well, counting drops and stirring until the pink color fades away. Record the number of drops required to reach this endpoint.
7. Repeat this procedure for wells # 2 and 3.
8. Check your data. All three titrations should agree within two drops of one another. If they do not, repeat the titration.
9. Clean your Beral pipet and your 24 well plate with distilled water.

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1. Look up in the handbook the solubilities of Ca(OH)₂ in water as temperature varies. Explain in terms of energy the reasons for this trend. Does your answer fit this trend?
2. Why is the hydroxide concentration squared in the K_sp?
3. Distinguish between the expressions dilute, concentrated, and saturated. Which could be at a condition of equilibrium?
   

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Data Analysis

1. Calculate the average number of drops of HCl used.
2. Calculate the molarity of the OH^- ion by using the formula: Ma x Va = Mb x Vb
3. Determine the molarity of calcium hydroxide.
4. Calculate the Ksp of calcium hydroxide. Use K_sp = [Ca²⁺] [OH^-]²
5. Calculate the molarity of a saturated Ca(OH)₂ solution in moles/liter.
6. Calculate the concentration of a saturated Ca(OH)₂ solution in grams/liter.
7. Determine the concentration of a Ca(OH)₂ saturated solution in grams/100 mL

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Safety

Calcium hydroxide is caustic and hydrochloric acid is corrosive; both can cause blindness. Wear eye protection at all times. Have an eye wash available.

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1. 1.85 g/L at 0 °C; 0.77 g/L at 100 °C).
2. The hydroxide concentration is squared in the K_sp determination as there are twice as many hydroxide particles (ions) as calcium ions. (The literature value of K_sp depends on the reference source and the temperature. A value of 7.9 x 10^-6 at 25 °C was found in a recent chemistry text.)
3. The expressions dilute, concentrated, and saturated describe the amount of solute in a solution. A solution can only be at equilibrium with undissolved solute in a saturated solution. (A saturated solution may be dilute or concentrated.)
   

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Calculate the average number of drops of HCl used.

(27 + 28 + 28)/3 = 28

Calculate the molarity of the OH- ion by using the formula

Ma x Va = Mb x Vb

(0.10 M) (28 drops) = (MOH^-) (50 drops)

(MOH^-) = (0.10 x 28)/(50) = 0.056 M

Determine the molarity of calcium ion.

Ca(OH)₂ --> Ca²⁺ + 2 OH^-

M Ca²⁺ = 0.5 x (MOH^-) = 0.5 x 0.056 = 0.028 M

Calculate the molarity of a saturated Ca(OH)₂ solution in mol/liter. It is the same as the M Ca²⁺, or 0.028 M.
Calculate the Ksp of calcium hydroxide

Ksp = [Ca²⁺] [OH^-]₂

= (0.028)(0.056)Û

= 8.8 x 10^-5

Calculate the concentration of a saturated Ca(OH)₂ solution in gram/liter.

(0.028 mol Ca(OH)₂ /L) x (74 g Ca(OH)2 / mol) = 2.07 g/L

Determine the concentration of a saturated solution in gram/100 mL.

(2.07 g/L) x (1 L/1000 cm³) x (100 cm^/3) = 0.21 g (per 100 cm³)

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Prepare the saturated solution the day before the lab then filter on the day of the lab so the students will have a clear solution.

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• pulled Beral pipet (or thin stem pipets cut to 1" and tipped with a plastic pipet delivery tip)
• 24 well plate
• tooth picks
• saturated solution of calcium hydroxide (5 g Ca(OH)₂ in 100 mL of distilled water; close to air; shake twice daily over a 1 week period; filter; store filtrate in tightly capped bottle as limewater or saturated calcium hydroxide)
• 1% phenolphthalein indicator (dissolve 1 g phenolphthalein in 60 mL of 95% ethanol; add enough distilled water to bring the total volume to 100 mL.)
• 0.1 M HCl (made by dissolving 9 mL of conc. HCl in 1 L of solution).

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The original version of this lesson was developed by:

Skip A. Anderson
Fremont Senior High
1750 North Lincoln
Fremont, NE 68025

Chris E. Dixon
Columbus Senior High
2200 26th St
Columbus, NE 68601

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