Cracking the SAT Chemistry Subject Test

Example:    Na+ (the    conjugate   acid    of  NaOH)   is  neutral.

2. 
   

        The conjugate   base    of  a   strong  acid    is  neutral.

   
   

   Example: Cl− (the conjugate base of HCl) is neutral.

   
   


3. The conjugate acid of a weak base is an acid.
   Example: NH 4 + (the conjugate acid of NH 3 ) is acidic.


4. The conjugate base of a weak acid is a base.
   Example: F− (the conjugate base of HF) is basic.

Memorize these conjugate rules. You’ll need to know them in order to make
sense of acid-base titration experiments.

Buffers

Buffers are solutions used to minimize (not prevent) a change in pH when an
additional acid or base is introduced into solution. Buffers are made out of
conjugate weak acids and bases—the acid/base pair must be conjugates because
if they weren’t, they would immediately react, neutralize one another, and fail to
establish a reversible reaction. Therefore, a buffer consists of a conjugate pair of
a weak acid and weak base.

Calculating the pH of Buffers

Thanks to the algebraic skills of Henderson and Hasselbalch, calculating the pH
of a buffer solution has been reduced to a relatively simple exercise in plug and
chug. The most common version of the Henderson-Hasselbalch equation is

pH  =   pKa +   log 

or