Question

When is the Henderson-Hasselbalch Equation used?

Answer 1

The Henderson-Hasselbalch equation is used when you are doing buffer calculations.

Explanation:

An acid buffer is a solution of a weak acid and its salt.

The equilibrium reaction is

`"HA + H"_2"O" ⇌ "H"_3"O"^+ + "A"^"-"`

`K_a = (["H"_3"O"^+]["A"^"-"])/(["HA"]]`

The Henderson-Hasselbalch equation is

`"pH" = "p"K_a + log((["A"^"-"])/"[HA]")`

There are four variables in this equation: `"pH", "p"K_a, ["A"^"-"]"`, and `["HA"]`.

You can calculate any one of them if you know the values of the other three.

Similarly, a basic buffer is a solution of a weak base and its salt.

The equilibrium reaction is

`"B + H"_2"O" ⇌ "BH"^+ + "OH"^"-"`

`K_b = (["BH"^+]["OH"^"-"])/(["B"]]`

The Henderson-Hasselbalch equation is

`"pOH" = "p"K_b + log ((["BH"^+])/(["B"]))`

There are four variables in this equation: `"pOH", "p"K_b, ["BH"^+]"`, and `["B"]`#.

You can calculate any one of them if you know the values of the other three.

You can calculate `"[pH](https://socratic.org/chemistry/acids-and-bases/the-ph-concept)"` from the relationship

`"pH" = "14.00 - pOH"`