Post on 23-Feb-2016
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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)
pH
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)
pH
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)
pH
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)
pH
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