Titrations and Buffered Solutions

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Titrations and Buffered Solutions

• Chapter 16. Sections 16.7-16.8

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POP QUIZ!

• Open Note!

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Pop Quiz Open Note

• What would be the OH- if the H is 2.6 x
  10-8?
• What is the pH of a solution that has a
  concentration of OH- 2.0 x 10 -2 M
• Calculate the pH value for a solution in which
  H 1.0 x 10-3
• The pOH is found to be 6.59, what is the H and
  OH-?
• Calculate the pH of a solution of 5.0 x 10-3 M
  HCl?

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Review

• What does pH measure?
• What makes something an acid? A base?
• What is the relationship between the strength of
  an acid or a base and its conjugate?

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Acid-Base Titrations

• As you know that a strong acid contains H ions
  in solution and a strong base contains OH- ions
  in a solution
• So the net ionic reaction in a strong acid base
  reactions is
• H(aq) OH-(aq) ? H2O(l)
• This reaction is called a neutralization reaction

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Acid-Base Titrations cont

• To analyze the acid or base content of an unknown
  solution, scientists often use a titration
• A titration is a technique in which a known
  solution is used to analyze the unknown solution
• A measured volume of a solution of a known
  concentration called a titrant is used to
  neutralize an unknown solution called an analyte

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Acid-Base Titrations cont

• If the analyte is an acid or base, titrant would
  be a standard solution, or a solution of known
  concentration
• The standard solution is loaded into a buret
• A buret is a long cylinder with a stopcock at the
  bottom and is graduated to tell you exactly how
  much liquid is in it

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Acid-Base Titrations cont

• From the buret, the titrant is added slowly to
  the unknown liquid, until enough on the titrant
  has neutralized all of the analyte.
• All of the H and OH- have reacted to form H2O
• This is called the stoichiometric point or the
  equivalence point for the titration
• You can tell when all has been neutralized by an
  indicator

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Acid-Base Titrations cont

• Example
• Lets say I have unknown concentration of HCl, but
  I have known concentration of NaOH.
• When the amount of H from the HCl has completely
  reacted with all of the OH- from the NaOH, then I
  will have a pH of 7.

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Acid-Base Titrations cont

• Determine The concentration of .25 liters HNO3
  that .050 L of .200 M NaOH neutralizes
• 1) what is the reaction?
• H OH- ? H2O
• 2) How much OH- do I have?
• .050 L x .200 M .010 mol OH-
• 3) How much H would I need to react
• with the OH-?
• .010 mol H
• 4) So what molarity?
• .010 moles/ ,25 liters .4 M

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Acid-Base Titrations cont

• You try one!
• Determine the volume of .250 M KOH required to
  titrated .025 L of .150 M HCl
• 1. Determine the reaction
• H OH- ? H2O
• 2. Find the moles of your known
• .025 L x .150 mole .00375 mole of H
• 3. Determine the moles needed of the unknown
• .00375 mol of OH-
• 4. Determine the volume needed
• M mol/ L or .250 M .00375 mol/ x
• x .015 L or 15 mL of KOH

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Acid-Base Titrations cont

• If you monitor the pH of the solution during the
  entire titration process and plot the pH, you
  will get a graph of the pH versus the volume of
  the titrant added.
• This is called a titration curve or pH curve.

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Buffered Solutions

• A buffered solution is a solution that resists a
  change in its pH even when a strong acid or base
  is added to it.
• Example When a small amount of HCl is added to
  pure water it can greatly change the pH, but when
  it is added to a buffered solution, it changes
  the pH only slightly.

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Buffered Solutions cont

• Buffer solutions consist of a weak acid and its
  conjugate base.
• The resistive action is the result of the
  equilibrium between the weak acid (HA) and its
  conjugate base (A-)
• HA(aq) H2O(l) ? H3O(aq) A-(aq)

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Buffered Solutions cont

• Buffered solutions are very important in our
  every day lives.

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Buffered Solutions cont

• Characteristics of a buffered solution
• The solution contains a weak acid and its
  conjugate base
• The buffer resists changes in pH by reacting with
  any H or OH- so that the ions do not accumulate
  and drastically change the pH
• Any acid reacts with the conjugate base
• Any base reacts with the weak acid

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Homework

• Read pages 524-529, problems 1-5 on page 529,
  problems 43 and 49 on page 535