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Another example. What is the pH of 0.100 M citric acid?. What are you thinking…?. I am thinking absolutely nothing. I am waiting for you to tell me what to think. I’m thinking it must be equilibrium because that’s all we talk about.

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Another example

Another example

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What is the ph of 0 100 m citric acid
What is the pH of 0.100 M citric acid?

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What are you thinking
What are you thinking…?

  • I am thinking absolutely nothing.

  • I am waiting for you to tell me what to think.

  • I’m thinking it must be equilibrium because that’s all we talk about.

  • I’m thinking it must be equilibrium because it is asking about the pH

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What is the ph of 0 100 m citric acid1
What is the pH of 0.100 M citric acid?

Citric acid is H3C6H5O7

Now, I’m thinking…

  • Must be an acid

  • Must be diprotic

  • Must be triprotic

  • Must be a strong acid

  • Must be a weak acid

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What is the ph of 0 100 m citric acid2
What is the pH of 0.100 M citric acid?

Ka1=7.1x10-4

Ka2=1.7x10-5

Ka3=4.1x10-7

Now, I’m thinking:

  • Must be a weak acid

  • Must be a strong acid

  • Must be a triprotic weak acid

  • I don’t get paid to think, you do.

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What is the ph of 0 100 m citric acid3
What is the pH of 0.100 M citric acid?

Ka1=7.1x10-4

Ka2=1.7x10-5

Ka3=4.1x10-7

Now, I’m thinking:

  • 3 parts

  • 2 parts

    D. I’ve had about enough of your nonsense.

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What is the ph of 0 100 m citric acid4
What is the pH of 0.100 M citric acid?

Ka1=7.1x10-4

Ka2=1.7x10-5

Ka3=4.1x10-7

H3C6H5O7 (aq) + H2O (l) ↔ H3O+ (aq) + H2C6H5O7- (aq)

H2C6H5O7- (aq) + H2O (l) ↔ H3O+ (aq) + HC6H5O72- (aq)

HC6H5O72- (aq) + H2O (l) ↔ H3O+ (aq) + C6H5O73- (aq)

Take them one at a time…or do I?

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Always try the assumption, we only have 30 seconds to lose.

Assume x<<0.100

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Check

close but no cigar

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Start where the first one leaves off
Start where the first one leaves off

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Always try the assumption, we only have 30 seconds to lose.

Assume x<<0.00808

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Check

YAY! It works.

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Start where the first one leaves off1
Start where the first one leaves off

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Let s take a moment for some deep reflection
Let’s take a moment for some deep reflection….

Did anything change during the second equilibrium?

Yes, EVERYTHING changed.

No, NOTHING changed.

Some things changed, some things didn’t

What are these “things” of which you speak?

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Nothing changed
Nothing changed…

…to 2 sig figs.

Which is a good thing! If it had changed, I would upset the first equilibrium!

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First equilibrium
First equilibrium

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Second equilibrium
Second equilibrium

They BOTH have to be satisfied if I’m truly at equilibrium.

Let’s pretend this second equilibrium turned out differently…

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Imaginary second equilibrium
Imaginary second equilibrium

Now, look back at the first equilibrium…two of these compounds are the same!

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Imaginary first equilibrium
Imaginary first equilibrium

It can’t be at equilibrium anymore.

The assumption that I can treat the equilibria separately relies on them not undoing each other. The bigger the K difference, the better. Otherwise, you have to solve both K’s simultaneously rather than consecutively.

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