The%20Guide%20to%20Buffer%20Problems

The%20Guide%20to%20Buffer%20Problems - The Guide to Buffer...

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The Guide to Buffer Problems A Buffer is a solution containing a weak acid and its conjugate base or a weak base and its conjugate acid. A buffer is used to control the pH of a solution. Because of the common ion effect the mixture of a weak acid and its conjugate base inhibits the ionization of both. A buffer also reduces changes in pH due to the addition of a strong acid or strong base. Strong acid added to a buffer reacts with the conjugate (weak) base to form more of the conjugate (weak) acid. Strong base added to a buffer reacts with the conjugate (weak) acid to form more of the conjugate (weak) base. Types of Buffer Problems There are three main types of buffer problems 1. Finding the pH of a buffer solution. 2. Finding the effect (pH change) of adding strong acid or base to a buffer. 3. Determining how to make a buffer with a specific pH. Each of these three types of problem with the most common variations will be discussed. Part I. Finding the pH of a buffer problems. To find the pH of a buffer solution it is easiest to use the Henderson – Hasselbalch equation. For a buffer made from a weak acid, HA and its conjugate base A - - a [A ] {conjugate base concentration} pH = pK + log [HA] {weak acid concentration} The most common type of problem just requires finding pK a from K a and then plugging in [A - ] and [HA]. Example 1 What is the pH of a buffer solution that is 0.30M hypochlorous acid, HOCl and 0.50M sodium hypochlorite, NaOCl? K a = 3.5 x 10 -8 for HOCl. pK a = -log(3.5 x 10 -8 ) = 7.46 ⎛⎞ ⎜⎟ ⎝⎠ - a [OCl ] 0.50 pH = pK + log = 7.46 + log = 7.68 [HOCl] 0.30 Common variations include 1. Problems requiring stoichiometry to determine acid and base concentrations. 2. Buffer solutions produced by neutralization reactions 1
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3. Buffers containing a weak base and its conjugate acid Example 2 – A problem requiring stoichiometry to determine concentrations What is the pH of a buffer solution produced by dissolving 30.0g of NaNO 2 in 500.0 mL of 0.40M HNO 2 ( K a = 4.5 x 10 -4 ) and then diluting to 1.00L.
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The%20Guide%20to%20Buffer%20Problems - The Guide to Buffer...

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