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Titration = process that determines the amount of an acid or base in solution

Standard Solution = substance of known concentration

3 Steps:

1) Obtain measured amount of unknown acid/base

2) Add indicator

3) Add measured volume of standard solution until indicator changes color

End Point =

Ex: Phenolphthalein changes from colorless to pink at pH 7–9

Equivalence Point =

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Unknown Acid solution with indicator

Standard solution is added in measured amounts

Color change shows neutralization.

- the end point and equivalence point are not necessarily the same

- indicators are selected so that the equivalence point and end point as are near to each as possible

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Titrations

3 Types:

1) Strong Acid – Strong Base Titrations

2) Weak Acid – Strong Base Titrations

3) Strong Acid – Weak Base Titrations

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Strong Acid – Strong Base Titrations

- pH at the equivalence point is 7!

Why?

Ex: HCl + NaOH H₂O + NaCl

STRONG

Neutral Salt!

- We must still use stoichiometry calculations dependent on the chemical equation and the amounts added

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Strong Acid – Strong Base Titrations

Ex: For the titration of 50.0 mL of 0.200 M HCl with 0.100 M NaOH calculate the pH of the solution at the following selected points of the titration:

NO NaOH has been added:

b) 10.0 mL of NaOH has been added:

c) 20.0 mL (total as opposed to additional) of NaOH has been added:

d) 50.0 mL (total) of NaOH has been added:

e) 100.0 mL (total) of NaOH has been added:

f) 200.0 mL (total) of NaOH has been added:

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pH Curve for Strong Acid – Strong Base Titrations

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Weak Acid – Strong Base Titrations

- pH at the equivalence point is NOT 7!

Ex: HC₂H₃O₂ + NaOH H₂O + NaC₂H₃O₂

WEAK

STRONG

Basic Salt!

- The pH at the equivalence point will be GREATER than 7 due to the presence of the basic salt!

- What have we created if there is a weak acid and the salt of its conjugate base present in the same solution…

A BUFFER!!!

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5 Points of Interest for Weak Acid – Strong Base Titrations

1) pH before the titration begins

2) pH on the way to the equivalence point

3) pH at the midpoint of the titration (½ equivalence point)

- treat as usual, RICE table time!

- the salt of the acid is now present… we’re in BUFFER land!

- use our buffer formula:

- pH = pK_a

Why?

½ of the acid has been converted to salt so, [acid] = [base]

- thus, [H⁺] = K_a x 1 …. pH = pK_a!

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5 Points of Interest for Weak Acid – Strong Base Titrations

4) pH at the equivalence point

5) pH after the equivalence point

- only the salt is present, so we are calculating the pH of the salt

- salt hydrolysis RICE fun!

- only the excess of strong base will matter

- calculate the molarity of excess base present and solve

(be careful of pOH and pH!)

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Weak Acid – Strong Base Titrations

Ex: Consider the titration of 50.0 mL of 0.10 M HC₂H₃O₂ solution

(K_a = 1.8 x 10^-5) with 0.10 M NaOH. Calculate the pH during the course of the titration for the specified volumes of NaOH added.

Case 1: Calculate the pH of the solution before NaOH is added.

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Weak Acid – Strong Base Titrations

Ex: Consider the titration of 50.0 mL of 0.10 M HC₂H₃O₂ solution

(K_a = 1.8 x 10^-5) with 0.10 M NaOH. Calculate the pH during the course of the titration for the specified volumes of NaOH added.

Case 2: Calculate the pH when 10.0 mL of NaOH is added.

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Weak Acid – Strong Base Titrations

Ex: Consider the titration of 50.0 mL of 0.10 M HC₂H₃O₂ solution

(K_a = 1.8 x 10^-5) with 0.10 M NaOH. Calculate the pH during the course of the titration for the specified volumes of NaOH added.

Case 3: Calculate the pH after addition of 25.0 mL of NaOH.

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Weak Acid – Strong Base Titrations

Ex: Consider the titration of 50.0 mL of 0.10 M HC₂H₃O₂ solution

(K_a = 1.8 x 10^-5) with 0.10 M NaOH. Calculate the pH during the course of the titration for the specified volumes of NaOH added.

Case 4: Calculate the pH after addition of 50.0 mL of NaOH.

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Weak Acid – Strong Base Titrations

Ex: Consider the titration of 50.0 mL of 0.10 M HC₂H₃O₂ solution

(K_a = 1.8 x 10^-5) with 0.10 M NaOH. Calculate the pH during the course of the titration for the specified volumes of NaOH added.

Case 5: Calculate the pH after addition of 60.0 mL of NaOH.

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pH Curve for Weak Acid – Strong Base Titrations

- How does this differ from a strong-strong titration?

- What are some important points to note on this graph?

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Strong Acid – Weak Base Titrations

- pH at the equivalence point is NOT 7!

Ex: HCl + NH₄OH H₂O + NH₄Cl

STRONG

WEAK

Acidic Salt!

- The pH at the equivalence point will be LESS than 7 due to the presence of the acidic salt!

- What have we created if there is a weak base and the salt of its conjugate acid present in the same solution…

A BUFFER!!!

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5 Points of Interest for Strong Acid – Weak Base Titrations

1) pH before the titration begins

2) pH on the way to the equivalence point

3) pH at the midpoint of the titration (½ equivalence point)

- treat as usual, RICE table time!

- the salt of the base is now present… we’re in BUFFER land!

- use our buffer formula:

- pH = pK_a

Why?

½ of the base has been converted to salt so, [acid] = [base]

- thus, [H⁺] = K_a x 1 …. pH = pK_a!

be careful with K_a and K_b!

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5 Points of Interest for Strong Acid – Weak Base Titrations

4) pH at the equivalence point

5) pH after the equivalence point

- only the salt is present, so we are calculating the pH of the salt

- salt hydrolysis RICE fun!

- only the excess of strong acid will matter

- calculate the molarity of excess acid present and solve

(be careful of pOH and pH!)

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Strong Acid – Weak Base Titrations

Ex: Consider the titration of 100.0 mL of 0.050 M NH₃ solution

(K_b = 1.8 x 10^-5) with 0.10 M HCl. Calculate the pH during the course of the titration for the specified volumes of HCl added.

Case 1: Calculate the pH of the solution before HCl is added.

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Strong Acid – Weak Base Titrations

Ex: Consider the titration of 100.0 mL of 0.050 M NH₃ solution

(K_b = 1.8 x 10^-5) with 0.10 M HCl. Calculate the pH during the course of the titration for the specified volumes of HCl added.

Case 2: Calculate the pH when 10.0 mL of HCl is added.

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Strong Acid – Weak Base Titrations

Ex: Consider the titration of 100.0 mL of 0.050 M NH₃ solution

(K_b = 1.8 x 10^-5) with 0.10 M HCl. Calculate the pH during the course of the titration for the specified volumes of HCl added.

Case 3: Calculate the pH after addition of 25.0 mL of HCl.

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Strong Acid – Weak Base Titrations

Ex: Consider the titration of 100.0 mL of 0.050 M NH₃ solution

(K_b = 1.8 x 10^-5) with 0.10 M HCl. Calculate the pH during the course of the titration for the specified volumes of HCl added.

Case 4: Calculate the pH after addition of 50.0 mL of HCl.

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Strong Acid – Weak Base Titrations

Ex: Consider the titration of 100.0 mL of 0.050 M NH₃ solution

(K_b = 1.8 x 10^-5) with 0.10 M HCl. Calculate the pH during the course of the titration for the specified volumes of HCl added.

Case 5: Calculate the pH after addition of 60.0 mL of HCl.

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pH Curve for Stron Acid – Weak Base Titrations

- How does this differ from all of the graphs we have seen

thus far?

- What are some important points to note on this graph?