Chapter 16

Acid-Base Titration and Ph

Objectives

Describe the self-ionization of water.

Define pH, and give the pH of a neutral solution at 25o C.

Explain and use the pH scale.

Given [H3O+] or [OH-], find pH.

Given pH, fine [H3O+] or [OH-].

  1. Section 16-1 Aqueous Solutions and Concept of pH
    1. Hydronium Ions and Hydroxide Ions
      1. acids and bases by definition produce H3O+ and OH- ions.
      2. Water will do the same. Remember it is amphoteric.
    2. Self-ionization of water
      1. Definition- two water molecules produce a hydronium ion and a hydroxide ion by transfer of a proton.
      2. Reaction

        3. H2O(l) + H2O(l) « H3O+(aq) + OH-(aq)

         

         

         

      3. The molarity of hydronium ions and hydroxide ions measured by conductivity is 1.0 X 10-7mol/L
      4. The symbol for concentration is [ ]
      5. The ion concentration for water
        1. [H3O+] = 1.0 X 10-7mol/L
        2. [OH-] = 1.0 X 10-7mol/L
        3. This is expressed mathematically by the ionization constant of water
          1. Symbol Kw
          2. Equation Kw =[H3O+] + [OH-]
          3. Kw for water 1 X 10 –14 mol/L
    3. Neutral, Acidic, and Basic Solutions
      1. Neutral solution is one where

        2. [H3O+] = [OH-]

      2. Acidic solutions
        1. [H3O+] > [OH-]
        2. [H3O+] = 1 X 10 –5 mol/L
        3. [OH-] =1 X 10 –9 mol/L
      3. Basic solutions
        1. [H3O+] < [OH-]
        2. [H3O+] = 1 X 10 –9 mol/L
        3. [OH-] =1 X 10 –5 mol/L
      4. Calculation [H3O+] and [OH-]
        1. Equation

          2. 1 X 10 –14 mol/L =[H3O+] + [OH-]

        2. Practice
    4. The pH scale
      1. pH
        1. The pH of a solution is defined as the negative of the common logarithm of the hydronium ion concentration
        2. pH = - log[H3O+]

         

      2. pOH
        1. The pOH of a solution is defined as the negative of the common logarithm of the hydroxide ion concentration.
        2. pOH = -log [OH-]
      3. pH + pOH = 14.0

        4.  

      4. Calculations
        1. pages 487,488,489,490

     

     

     

     

     

     

     

    Objectives

    Describe how an acid-base indicator functions.

    Explain how to carry out an acid-base titration.

    Calculate the molarity of a solution from titration data.

  2. Section 16-2 Determining pH and Titrations
    1. Indicators and pH Meters
      1. Acid-base indicators are compounds whose colors are sensitive to pH.
      2. The pH range over which an indicator changes color is called its transition interval.
      3. pH meter determines the pH of a solution by measuring the voltage between the two electrodes that are placed in the solution.
      4. Table 16-6 shows color ranges of various indicators used in titrations

       

       

       

       

       

       

       

    2. Titration
      1. Neutralization reactions occur between acids and bases
      2. Neutralization reactions produce water
      3. Titration is a neutralization reaction
        1. Titration is the controlled addition and measurement of the amount of a solution of known concentration required to react completely with a measure amount of a solution of unknown concentration.
        2. Equivalence point is the point at which the two solutions used in a titration are present in chemically equivalent amounts.
          1. Important
          2. If we know the measured amount we can determine the unknown.
        3. End point is the point at which an indicator changes color.
        4. Titrations do not have to produce neutral solutions
          1. Neutral
            1. Strong base + strong acid
            2. Figure 16.9 page 499
          2. Acidic salt or basic salt
            1. Strong acid + weak base
            2. Strong base + weak acid
            3. Figure 16.9 page 499
    3. Molarity and Titration
      1. Standard solution- a solution that contains the precisely known concentration of a solute.
      2. The known can be used to determine the molarity of another solution by titration.
      3. Problem steps
        1. Start with the balanced equation for the neutralization reaction, and determine the chemically equivalent amount of the acid and base.
        2. Determine the moles of acid (or base) form the known solution used during the titration.
        3. Determine the moles of solute of the unknown solution used during the titration.
        4. Determine the molarity of the unknown solution.