CHM 2046C Module 12 Name: ____ Answers_____

J. Determining K_sp from a Salt’s Solubility 10 points

Given the following insoluble salts, estimate the salt’s molar solubility from the Ksp :

1. Calcium hydroxide, Ca(OH)₂ (s) dissolves in water to the extent of 1.04 g/L. What is the value of the K_sp?

First write the dissociation reaction:

. Ca(OH)₂ (s) + HOH (l) ß à Ca²⁺ (aq) + 2 OH^1- (aq)

Setup the K_sp expression for the reaction:

K_sp = [Ca²⁺][OH^1-]²

Now you need to change the gram solubility to molar solubility. The molar mass of Ca(OH)₂ is 40.1 + 32.0 + 2.0 = 74.1 g/mol.

1.04 g		1 mol
-------	x	--------	=	0.0133 mol/L
1 L		74.1 g

If the molar solubility of Calcium hydroxide is 0.0133 M, then the

[Ca²⁺] = 0.0133 M and the [OH^1-] is twice the Calcium or

[OH^1-] = 0.0266 M

Substituting into the Ksp expression:

K_sp = [0.0133][0.0266]²

The Answer: K_sp = 9.4 x 10 ^-6

(Note this is slightly lower than the accept value in the back of the text)

2. You place 1.834 g of Ca(OH)₂ (s) in 1.00 L of pure water at 25^oC. The pH of the solution is found to be 12.68. What is the value of

the K_sp ?

When you place 1.834 g of Calcium hydroxide in water, there are three probabilities:

(1) It all dissolves and there is no equilibrium

(2) It all dissolves except 1 granule so there is an equilibrium

(3) Only a part of the solid dissolves and there is an equilibrium

In all three cases, since you do not know how much dissolves you can not use the 1.834 g amount in the calculation.

Again write the dissociation reaction for Calcium hydroxide:

. Ca(OH)₂ (s) + HOH (l) ß à Ca²⁺ (aq) + 2 OH^1- (aq)

Setup the K_sp expression for the reaction:

K_sp = [Ca²⁺][OH^1-]²

The pH is given at 12.68.

Therefore the pOH is 14.00-12.68 or 1.32

Since pOH = - log [OH^1-] = 1.32

The [OH^1-] = 10^{-1.32 or} 0.048 M

The [Ca²⁺] is one half the [OH^1-]

[Ca²⁺] = 0.024

Substituting into the K_sp Expression:

K_sp = [0.024][0.048]²

The Answer:

K_sp = 5.5 x 10^-5

This is the K_sp table value in the back of the book.

McMurray Reference:

Read Section 15.11. Look at Worked Examples 15.8 and 15.9 on Page 520.

Do problems 15.21 and 15.22 page 621.

Also do additional end of chapter problems 15.92, 15.93,and 15.94 pages 641-642.

Kotz Reference:

Chapter 18 Section 18.4 p873-875

Section 18.4: Relating Solubility and K_sp pages 873-875.

Review worked example 18.8 pages 875. Work Exercise 18.12 page 875. At the end of the chapter there are six additional exercises: #39-#44 page 896.

Additional Worked Examples from another book:

Another Worked Example: Determine if it precipitates: