Weak Acids and Bases

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Weak Acids and Bases
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Acids and Bases Basic Concepts
Weak Acids
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Acids and Bases Basic Concepts
Weak Acids

• The concentration of the un-ionized acid is
  always the greatest of the three concentrations.

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Acids and Bases Basic Concepts
Weak Bases

• Ammonia is a weak base because most of its
  molecules dont react with water to form ions.

• Other examples of bases that produce so few OH
  ions that they are considered to be weak bases
  are Al(OH)3, and Fe(OH)3.

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Acids and Bases Basic Concepts
Strength Is Not Concentration

• Although the terms weak and strong are used to
  compare the strengths of acids and bases, dilute
  and concentrated are terms used to describe the
  concentration of solutions.

• The combination of strength and concentration
  ultimately determines the behavior of the
  solution.

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pH of weak acids and bases

• Finding pH of strong acids and bases is
  simplemultiply concentration of acid or base by
  the number of acidic hydrogens or hydroxide ions,
  then take the negative log.
• Weak acids and bases are a little more
  complicated. They do not dissociate completely,
  so one must decide what amount is dissociated.
  This is done using the Ka value.
• To do this, one must consider the net ionic
  reaction
• HC2H3O2 H2O(l) ? C2H3O2- H3O

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• In this reaction, acetic acid reacts with water
  to form the acetate ion (Conjugate base) and the
  hydronium ion.
• If you recall from last chapter, you can write an
  equilibrium expression by dividing products by
  reactants.
• So KaC2H3O2-H3O
• _______________
• HC2H3O2

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• So all you do is substitute in and solve for the
  hydronium ion, which allows you to find pH
• Bases are the same, except you use Kb.
• KbOH-Conjugate acid
• ______________________
• base

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Practice question 1

• Find the pH of 0.05 M acetic acid-HC2H3O2 (Ka
  1.8 x 10-5 )

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• Find pOH of 0.5M aniline C6H5NH2 (kb3.8 x 10-10)

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• Find pH of 2.00 M nitrous acid (HNO2). Ka4.0 x
  10 -4

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Acids and Bases Additional Concepts
Acid-Base Titrations

• The general process of determining the molarity
  of an acid or a base through the use of an
  acid-base reaction is called an acid-base
  titration.

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Acids and Bases Additional Concepts
Acid-Base Titrations

• The known reactant molarity is used to find the
  unknown molarity of the other solution.

• Solutions of known molarity that are used in this
  fashion are called standard solutions.

• In a titration, the molarity of one of the
  reactants, acid or base, is known, but the other
  is unknown.

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Acids and Bases Additional Concepts
Acid-Base Titrations

• You know that NaOH and HCl react completely.

• You know the concentration of the NaOH solution,
  so it is your standard solution.

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Acids and Bases Additional Concepts
Acid-Base Titrations

• You can use the reaction, the volumes of acid and
  base used, plus the molarity of the base to
  determine the molarity of the unlabeled HCl.

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• To do this, simply use the following formula
• HVacidOH-Vbase