Homework answers / question archive / SCH4U Unit 4 - Activity 8- Ksp Common ion effect reading LAB Determining Ksp for Calcium Hydroxide Introduction: Calcium hydroxide, Ca(OH) (s), is a slightly soluble salt that dissolves according to the chemical 2 reaction: 2+          1- Ca(OH) (s)    Ca (aq) + 2OH (aq)   2 In this experiment, the solubility product constant of calcium hydroxide, Ca(OH) (s), is determined by 2 mixing a fixed volume of 0

SCH4U Unit 4 - Activity 8- Ksp Common ion effect reading LAB Determining Ksp for Calcium Hydroxide Introduction: Calcium hydroxide, Ca(OH) (s), is a slightly soluble salt that dissolves according to the chemical 2 reaction: 2+          1- Ca(OH) (s)    Ca (aq) + 2OH (aq)   2 In this experiment, the solubility product constant of calcium hydroxide, Ca(OH) (s), is determined by 2 mixing a fixed volume of 0

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SCH4U Unit 4 - Activity 8- Ksp Common ion effect reading

LAB

Determining Ksp for Calcium Hydroxide

Introduction:

Calcium hydroxide, Ca(OH) (s), is a slightly soluble salt that dissolves according to the chemical

2

reaction:

2+          1-

Ca(OH) (s)    Ca (aq) + 2OH (aq)   2

In this experiment, the solubility product constant of calcium hydroxide, Ca(OH) (s), is determined by

2 mixing a fixed volume of 0.100 mol/L sodium hydroxide with serial dilutions of aqueous calcium nitrate in a series of spot plate wells.  The Ksp of calcium hydroxide is determined by calculating the product of the concentrations of the ions in the well containing the highest concentration of ions with no visible precipitate.

Purpose:

To determine the Ksp of calcium hydroxide

Materials:

0.100 mol/L NaOH     spot plate        water

0.100 mol/L Ca(NO ) 4  pipets

3 2

 

Observation:

well

#

A

Initial [Ca

2+

)

aq

(

]

B

Initial [OH

1-

(

)

aq

]

C

Final

Ca

[

2+

)

(

aq

]

D

Final

OH

[

1-

(

aq

)

]

E

(

)

C x D

ion product

1

0.1

0.1

0.05

0.05

0.000125

2

0.05

0.1

0.025

0.05

0.0000625

3

0.025

0.1

0.0125

0.05

0.00003125

4

0.0125

0.1

0.00625

0.05

0.000015625

5

0.00625

0.1

0.003125

0.05

0.000007812

6

0.003125

0.1

0.0015625

0.05

0.000003906

7

0.0015625

0.1

0.00078125

0.05

0.000001953

8

0.00078125

0.1

0.000390625

0.05

0.000000976

 

9. 0.0003906250.1    0.0001953120.05   0.000000488
10. 0.0001953120.1    0.0000976560.05   0.000000244
11. 0.0000976560.1    0.0000488280.05   0.000000122 12             0.0000488280.1      0.0000244140.05           0.000000061

Using the procedure, complete the observation table below.  Keep 3 significant digits. for concentration. Determine the ion product to the nearest thousandth.

Procedure:

1. Add 5 drops of water to each of 11 consecutive wells in Row A of the spot plate (wells A2 to A12), leaving the first well, A1, empty.
2. Add 10 drops of 0.100 mol/L calcium nitrate to well A1.
3. Draw the solution from A1 into an empty pipet, and place 5 drops of the solution into A2.  (Return any excess solution to the first well).
4. Using the same pipet, transfer 5 drops of the solution in A2 to the water in A3.
5. Repeat step 4 for each of the remaining wells.  Discard the 5 drops from well A12 into the sink with lots of running water.
6. Using a new pipet, place 5 drops of 0.100 mol/L sodium hydroxide solution into wells B1 through B12.
7. Use a clean pipet to transfer the entire contents of well A1 into well B1.  Mix well with the tip of the pipet.  Continue this process for wells A2 into B2, A3 into B3, etc.,  until A12 and B12 have been mixed.
8. Examine all of the wells. Identify the first well that appears to have no precipitate.
9. Dispose of the chemical wastes as instructed by your teacher.

Calculations: (Keep 3 significant digits for all concentrations)

1. Calculate the [Ca²⁺_(aq)]  in each well of row A.  Record these in column A of the table.
2. Record the initial [OH^1-_(aq)]  in each well of row B.  Record these in column B of the table.
3. Determine the [Ca²⁺_(aq)] and [OH^1-_(aq)] in wells B1 to B12, and record these values in columns C and D.
4. Calculate the ion product for the contents of each well.  Record these values in column E of the table.

Discussion:

1. a) Write the K_sp expression for calcium hydroxide.

b) Based on your observations of the first well that appears to have no precipitate, state     the ion product that determines the value of the K_sp in this experiment.

2. What is your experimental value for the K_sp for calcium hydroxide?  Hint:  Decide which ion product in column E of the table corresponds to the K_sp for calcium hydroxide.
3. Use your experimental value of K_sp to determine the solubility of calcium hydroxide.
4. The value in reference books as the solubility product for calcium hydroxide is 7.9 x 10^-6.  Calculate the percent difference by comparing your value and the accepted value. ( see text p 778).
5. Identify sources of error and uncertainty in the procedure of this experiment. Suggest two improvements.