Question

Salicylic acid has a Ka of 1.1 x 10^-3. Calculate the ratio of the concentration of salicylic acid and its sodium salt in a solution that has a pH of 2.50?

Answer 1

`(["HA"])/(["A"^"-"]) = 2.87`

Explanation:

The equation for the equilibrium is

`"HA + H"_2"O" ⇌ "H"_3"O"^"+" + "A"^"-"; K_text(a) = 1.1 × 10^"-3"`

To get the `(["HA"])/(["A"^"-"])` ratio, we use the Henderson-Hasselbalch equation:

`color(blue)(bar(ul(|color(white)(a/a)"pH" = "p"K_text(a) + log((["A"^"-"])/(["HA"]))color(white)(a/a)|)))" "`

`"pH - p"K_text(a) = log((["A"^"-"])/(["HA"]))`

`log((["HA"])/(["A"^"-"])) = "p"K_text(a)color(white)(l) "- pH" = "-log"(1.1 × 10^"-3")color(white)(l) "- 2.50" = "2.96 - 2.50" = 0.46"`

`(["HA"])/(["A"^"-"]) = 10^0.46 = 2.87`