Buffers

What is a buffer solution?

solution that RESISTS changes of pH

What are buffer solutions made of?

Weak acid w/ associated salt (CONJUGATE BASE)

Weak base w/ associated salt (CONJUGATE ACID)

What are the TWO assumptions made in buffers?

All A- (aq) ions come from the salt

Almost all HA molecules put in the buffer stays unchanged

What happens when a small amount of acid is added to a buffer?

More H3O+ (aq) added

Le Chatelier’s principle : A- from salt reacts w/ H3O+

Equilibrium moves to the left

What happens when a small amount of base is added to buffer?

OH- (aq) added

Le Chatelier’s Principle : OH- reacts w/ H3O+

Equilibrium moves right

• more HA reacts w/ H2O to form more H3O+

What is the Henderson-Hasselbalch equation?

pH = pKa + log [CONJU BASE]/[ACID]

From the equation, what are 2 factors that the pH of the buffer solution depend on?

1. Ka value— gives ‘coarse tuning’ of buffer pH

   • weak acid/base determines region of pH the buffer is in

2. Ratio of conju base and acid— gives ‘fine tuning’ of buffer pH