4.12 The Effect of pH on Solubility

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From the course by University of Kentucky

Advanced Chemistry

326 ratings

University of Kentucky

Advanced Chemistry

326 ratings

A chemistry course to cover selected topics covered in advanced high school chemistry courses, correlating to the standard topics as established by the American Chemical Society. Prerequisites: Students should have a background in basic chemistry including nomenclature, reactions, stoichiometry, molarity and thermochemistry.

From the lesson

Aqueous Equilibria

This unit continues and expands on the theme of equlibria. You will examine buffers, acid/base titrations and the equilibria of insoluble salts.

4.01 Buffers and the Common Ion Effect10:01

4.02 pH of Buffer Solutions14:04

4.02a Aqueous Equilibria Worked Example 13:58

4.03 Buffer Action25:21

4.03a Aqueous Equilibria Worked Example 25:22

4.03b Aqueous Equilibria Worked Example 35:11

4.04 Buffer: Preparation and Capacity8:45

4.05 Strong Acid - Strong Base Titration17:20

4.06 Titrations Involving Either a Weak Acid or a Weak Base36:45

4.06 Part 1 Aqueous Equilibria Worked Example3:46

4.06 Part 1.a - Aqueous Equilibria Worked Example4:06

4.06 Part 1.b - Aqueous Equilibria Worked Example4:55

4.06 Part 1.c - Aqueous Equilibria Worked Example4:21

4.06 Part 1.f Aqueous Equilibria Worked Example8:01

4.06 Part 1.h Aqueous Equilibria Worked Example3:57

4.06 Part 2.a Aqueous Equilibria Worked Example8:25

4.06 Part 2.b Aqueous Equilibria Worked Example5:53

4.07 Polyprotic Acid Titrations5:53

4.08 Indicators10:19

4.09 Solubility Equilibria4:50

4.10 Molar Solubility and the Solubility Product10:37

4.10.a Aqueous Equilibria Worked Example2:20

4.11 Molar Solubility and the Common Ion Effect7:20

4.11 2.b - Aqueous Equilibria Worked Example4:42

4.11 2.c Aqueous Equilibria Worked Example3:45

4.12 The Effect of pH on Solubility7:07

4.12.b Aqueous Equilibria Worked Example3:42

4.12c Aqueous Equilibria Worked Example3:30

4.13 Precipitation Reaction and Selective Precipitation15:03

4.13a Aqueous Equilibria Worked Example9:05

Meet the Instructors

Dr. Allison Soult
Lecturer
Chemistry
Dr. Kim Woodrum
Sr. Lecturer
Chemistry

In the previous learning objective we learned that

the common ion always decreases

the solubility of an insoluble salt.

In this learning objective the solubility of an insoluble salt.

In this learning objective

when and how pH affects

the solubility of a salt.

This is going to occur if

one of the ions in the salt

is an acid or a base.

So here is an example.

This will be a very simplistic example we will

expand it to more complicated examples.

Magnesium hydroxide is

fairly insoluble.

But it itself is a base

and we see that we have that OH-.

So the anion of this salt

is a base.

So if we look at that equilibrium

and we just consider Le Chatelier's principle

if we had the system at equilibrium and we just consider Le Chatelier's principle

if we had the system at equilibrium

and we when in and we added a base

to that system

would we say

thinking about which way the equilibrium would shift

that the solubility of the Magnesium Hydroxide would increase

or the solubility would decrease?

Well, if you said it would decrease you are correct.

Why would it decrease? Well, if you

add OH-

anytime you add something on the right hand

side it will shift to the left.

To try and use it up. side it will shift to the left.

To try and use it up.

You are increase the solid

that would be a representation of a

decrease in solubility.

So what is adding

base, adding a base is the pH

would be going up.

As you add that base, then that would always

that would always shift this salt to the left.

So lets do the opposite. that would always shift this salt to the left.

So lets do the opposite.

Lets add an acid.

Now when you add an acid, the

H_3O+

concentration goes up.

What does the OH- concentration do?

Think about that

and then answer the question.

Well, if you said it would

increase the solubility you would be correct.

Why is that?

Well, when the H_3O+ goes up by adding an acid

always the OH- goes down

because that product has to equal K_w.

If that is going down then this

is being removed. It is decreasing.

And you are going to shift it to try and produce more. is being removed. It is decreasing.

And you are going to shift it to try and produce more.

SO in this case, as you decrease

the OH- concentration by adding an acid

or in other words as the

pH goes down

the solubility will increase.

OK so lets consider another equilibrium.

Here is an insoluble salt.

For this substance it might quite be as obvious

as to why the pH would affect the

solubility of this salt.

But F- is the conjugate

base of a weak acid.

Our weak acid would be HF

and this is the conjugate base

of HF.

As a base it will be involved in

its own equilibrium, so here it is

F- in water, which is an aqueous solution

and we end up with

the F- having an OH

in its equilibrium.

Now this equilibrium here

obviously going to be affected by pH because

as we change the pH we are

changing the amount of OH-.

So lets walk our way through this

consideration with some questions.

So both of these equilibria are involved.

We have got the lead fluoride, an insoluble salt dissolving.

We have the F- of that salt

undergoing ionization with water. We have the F- of that salt

undergoing ionization with water.

So if we add a base

to this solution

what affect will that have to this solution

what affect will that have

on equilibrium number 2?

Well did you say it would shift it to the left?

That is correct. If you are adding Well did you say it would shift it to the left?

That is correct. If you are adding

a base, in other words

as the pH is going up

it is going to shift that

second equilibrium it is going to shift that

second equilibrium

to the left.

And that would increase the amount of F- in solution.

If you are increasing the amount of F- in solution

how would the increase of the concentration of the F-

affect the first equilibrium.

Think about that and answer

Well, did you say that would shift it to the left?

You would be correct. So if we add Well, did you say that would shift it to the left?

You would be correct. So if we add

if the F- is going up, it shifts it to the left. You would be correct. So if we add

if the F- is going up, it shifts it to the left.

so what would we say

the solubility of lead fluoride?

When we add a base and the pH is going up?

That would decrease the solubility.

Anything you get more solid in there then you started with

that is a decrease in solubility. Anything you get more solid in there then you started with

that is a decrease in solubility.

Now we looked at it in terms of adding

base, we looked at it in terms of

increasing the pH

and we could go through these

equilibria for adding an acid.

If you add and acid, that decreases the OH- equilibria for adding an acid.

If you add and acid, that decreases the OH-

if you decrease the OH- that shift it to the right

that decreases the F- concentration if you decrease the OH- that shift it to the right

that decreases the F- concentration

then we go to the first equilibrium. When that decreases the F- concentration

then we go to the first equilibrium. When

we go to the first equilibrium when the

F- is decreasing that would shift it

to the right. As it shifts it to the right

increase the solubility.

So you could go through and think about this for every salt

that you come across, but there is a bottom line.

The bottom line is this.

If you look at a salt

and the anion is a conjugate base of a weak acid

which we have.

Anion is a conjugate base of a weak acid.

Then the solubility will always increase

with decreasing pH.

In other words, if you added and acid.

So anytime to wanted to increase the solubility of a salt

if you look at the anion

and it is the conjugate base of a weak acid

then you would be able to

increase the solubility

by adding acid. Bringing the pH down will

increase the solubility of that insoluble salt.

i want to show you in this demonstration

how you can take an insoluble salt

and redissolve it by adding an

acid and lowering the pH.

But first I want to create an insoluble salt. acid and lowering the pH.

But first I want to create an insoluble salt.

I got magnesium sulfate in this test tube

and magnesium sulfate is very soluble.

But I am going to add some ammonia.

This is going to produce a precipitate

of magnesium hydroxide.

So I am going to produce the magnesium hydroxide.

Now you can see that

the test tube is getting cloudy Now you can see that

the test tube is getting cloudy

and that cloudiness is the

formation of the magnesium hydroxide.

Since the anion of magnesium hydroxide is the

conjugate base of a weak acid Since the anion of magnesium hydroxide is the

conjugate base of a weak acid

we will be able to redissolve the

magnesium hydroxide by adding an acid.

I am adding hydrochloric acid here. magnesium hydroxide by adding an acid.

I am adding hydrochloric acid here.

As we lower the pH

the magnesium hydroxide re-dissolves.

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