Henderson-Hasselbalch equation?

Question

Henderson-Hasselbalch equation= pH = pKa + log10([A–]/[HA])

We use this equation in order to determine what type of buffer solution is needed to render a target pH. Suppose that we have chosen an acid-base pair with a pKa value that is just slightly less than the desired pH value. Which of the following best describes what we have to do to “fine tune” our buffer solution?

1. Adjust the buffer solution so that [A–] = [HA].

2. Increase the [A–].

3. Decrease the [A–].

4. Increase the [HA].

Answer 1

Increase the conjugate base...the [A-]

Answer 2

Lancenigo di Villorba (TV), Italy

Your question is a VERY INTERESTING ONE!!
You wrote about a Buffer Solution, e.g. it may be an aqueous solution of a Weak Acid in presence of One Its Salt.
You said that the involved Acidic Stuff shows a pKa's VALUE SLIGHTLY LOWER THAN pH's VALUE DESIRED.
Thus, you ask :"HOW DO I ACT IN ORDER TO MODIFY pH's BUFFER AS I EXPLAINED?".

Hypotheses 1.
I pursue the following condition

|A-| = |HA|

since there is a Weak Acid in aqueous solutions, I back upon the Equilibrium's Constant

Ka = |A-| * |H+| / |HA|

so I may retrieve Ka = |H+|.....this is the Unsatisfactory Starting Condition!!!

Hypotheses 2.
I pursue the SALT's ADDICTION, e.g. I increase the |A-|'s Molarity. Since there is a Weak Acid in aqueous solutions, I back upon the Equilibrium's Constant

Ka = |A-| * |H+| / |HA|

so I MAY DIMINUTE |H+|'s Molarity.....THIS IS THE WAY!!!

Hypotheses 3.
I pursue the HEAVY METAL ION's ADDICTION, e.g. I diminute the |A-|'s Molarity. Since there is a Weak Acid in aqueous solutions, I back upon the Equilibrium's Constant

Ka = |A-| * |H+| / |HA|

so I MAY INCREASE |H+|'s Molarity.....THIS ISN'T THE WAY!!!

Hypotheses 4.
I pursue the WEAK ACID's ADDICTION, e.g. I increase the |HA|'s Molarity. Since there is a Weak Acid in aqueous solutions, I back upon the Equilibrium's Constant

Ka = |A-| * |H+| / |HA|

so I MAY INCREASE |H+|'s Molarity.....THIS ISN'T THE WAY!!!

I hope this helps you.

Answer 3

low pH = acidic solution.

so to fine tune a buffer you need to have the right ratio. a 1:1 has the pH = pKa so if you have more acid than conjugate base the pH will be lower than the pKa