Titrations

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Titrations
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Acid-Base Titrations

• Acid-base titration is a process for calculating
  the unknown concentration of an acid or base.

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Acid-Base Reactions

• H2C2O4(aq) 2 NaOH(aq) ? Na2C2O4(aq) 2
  H2O(l)
• acid base
• Carry out this reaction using a TITRATION.

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Apparatus for titrating an acid with a base
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Sample Problem

• In this sample titration, we are trying to
  determine the concentration of 20.00 mL of HCl.
• In the titration we will be neutralizing the HCl
  with 0.150 M NaOH.

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Step 1

• The NaOH, the titrant, is placed in the buret.
  The titrant is the solution of known
  concentration that is added from the buret.

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Step 2

• The HCl is placed in the Erlenmeyer flask along
  with approximately 20.00 mL of distilled water
  and 2-3 drops of phenolphthalein (which is used
  as an indicator).
• Since the solution in the flask is acidic,
  phenolphthalein is colourless.

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Step 3

• NaOH is added to the HCl in the flask.
• When the NaOH comes in contact with the solution
  in the flask, it turns pink and then the pink
  colour quickly disappears.
• This is because the OH- from the NaOH interacts
  with the phenolphthalein to change the
  phenolphthalein from colourless to pink.

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Why does the solution become clear?

• The solution becomes clear again as the hydronium
  ions from the hydrochloric acid neutralize the
  added hydroxide ions.
• As more NaOH is added, it takes longer for the
  pink colour to disappear.
• As it starts taking longer for the pink colour to
  disappear, the sodium hydroxide is added one drop
  at a time.

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Acid-Base Titration
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Equivalence Point

• The equivalence point of the titration is reached
  when equal numbers of moles of hydronium and
  hydroxide ions have reacted.
• When this happens in this titration, the pH of
  the solution in the flask is 7.0 and the
  phenolphthalein indicator is colourless.
• This would be a good time to stop, however the
  indicator is still colourless, so you must keep
  going.

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Titration Curve
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Step 4

• Add as little excess NaOH as possible.
• We want to add a single drop of NaOH to the
  colourless solution in the flask and have the
  solution in the flask turn pink and stay pink
  while the contents of the flask are swirled.
• This permanent colour change in the indicator is
  known as the endpoint of the titration and the
  titration is over.

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Sample Problem 1

• In an acid-base titration, 25.67 mL of 0.150 M
  sodium hydroxide, NaOH, was completely
  neutralized by 20.00 mL of hydrochloric acid,
  HCl.
• Calculate the concentration of the hydrochloric
  acid.

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Sample Solution

• Step 1
• Write the equation for the reaction
• HCl(aq) NaOH(aq) ? NaCl(aq) H2O(l)
• Step 2
• Determine the amount of moles of the titrant
  used.
• NaOHn 0.150 mol/L x 0.02567 L
• 3.85 x 10-3 mol NaOH

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• Step 3
• Using stoichiometric ratios, determine the
  concentration of HCl.
• Note It is a 11 mole ratio
• 3.85 x 10-3 mol 0.192 M HCl
• 0.02000 L

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Sample Problem 2

• In an acid-base titration, 17.45 mL of 0.180 M
  nitric acid, HNO3, was completely neutralized by
  14.76 mL of aluminium hydroxide, Al(OH)3.
• Calculate the concentration of the aluminium
  hydroxide.

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Sample Solution

• The balanced equation for the reaction is
• 3HNO3(aq) Al(OH)3(aq) ? Al(NO3)3(aq) 3H2O(l)
• The number of moles of nitric acid used is
• moles HNO3 0.180 mol/L x 0.01745 L
• 3.14 x 10-3 mol HNO3

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• Using the stoichiometric ratio of the reaction,
  the number of moles of aluminium hydroxide
  reacted is
• 3.14 x 10-3 mol HNO3 x 1 mol Al(OH)3 1.05 x
  10-3 mol Al(OH)3
• 3 mol HNO3
• Therefore, the concentration of the aluminium
  hydroxide is
• 1.05 x 10-3 mol Al(OH)3 0.0711 M Al(OH)3
• 0.01476 L

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Titration Curve of a Strong Acid with a Strong
Base.
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Which indicator(s) would you use for a titration
of HNO2 with KOH ?
Weak acid titrated with strong base.
At equivalence point, will have conjugate base
of weak acid.
At equivalence point, pH gt 7
Use cresol red or phenolphthalein
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Homework

• Read pages 599-602pay particular attention to
  the sample problems.
• Page 602 17-19