Acid/Base Titration

Acid–Base Titration• The concentration of a weak acid or a weak base in water is difficult – if not impossible – to measure directly.

• But we can calculate the concentration from the results of titration.

Acid–Base Titration• A titration is a carefully controlled neutralization rxn.

• To run a titration on a soln of unknown conc of an acid or a base, you need a 2nd soln called a standard soln.

• A standard soln contains acid or base in a known conc

Acid–Base Titration• If the unknown soln is an acid, the standard soln is a base–And vice a versa

• Another substance involved in a titration is an indicator.–which is a substance that changes color at a certain pH

Acid–Base Titration• Most of the time we use the common indicator Phenolphthalein.–Phenolphthalein is clear in acid

–Light pink in neutral–Dark pink in base

Procedure for Titration

• the standard is slowly added to the unknown soln

• the 2 solns mix, the acid in one neutralizes the base in the other

• Eventually, enough standard soln is added to neutralize all of the acid or base in the unknown soln.

Procedure for Titration

• The point at which this occurs is called the equivalence point.–[H30+] = [OH-]

• The point at which the indicator changes color is called the end point of the titration.

Procedure for Titration

• If the indicator is chosen correctly the end point is very close to the equivalence point.

• Therefore, at approximately the end pt of a titration the total mols of H+ donated by the acid is equal to the total mols of H+ accepted by the base.

Procedure for Titration

• For Arrhenius Acids and Bases we can say:

Total moles of H+ from the acid

Total moles of OH- from

the base=

• This is key to solving titration calcs

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0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 45.00milliliters NaOH (0.10 M)

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Weak Acid/Strong Base Titration

A solution that is 0.10 M CH3COOH is titrated with 0.10 M NaOH

Endpoint is above pH 7

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0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 45.00milliliters NaOH (0.10 M)

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Strong Acid/Strong Base Titration

A solution that is 0.10 M HCl is titrated with 0.10 M NaOH

Endpoint is at pH 7

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0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 45.00m illiliters HCl (0.10 M)

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Strong Acid/Strong Base Titration

A solution that is 0.10 M NaOH is titrated with 0.10 M HCl

Endpoint is at pH 7 It is important to

recognize that titration curves are not always increasing from left to right.

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0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 45.00milliliters NH3 (0.10 M)

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Strong Acid/Weak Base Titration

A solution that is 0.10 M HCl is titrated with 0.10 M NH3

Endpoint is below pH 7

Sample ProblemSolns of NaOH are used to

unclog drains. A 43 ml volume of NaOH was titrated with 32 ml

of .100M HCl. What is the molarity of the NaOH soln?

Analyze:• At the equivalence pt, the added acid has completely neutralized the unknown base.

• It took 32 ml of acid to neutralize 43 ml of base

• We know that mols of base = mols of acid, at the equiv pt

Solve:• moles of acid used:

–(.100mols/L)(.032L)= .0032mol of

acid• mols of acid = mols of base–.0032 mol base used in rxn.

• Molarity of NaOH–.0032mol/.043L = .074M