Question

Phenolphthalein is an indicator that turns from colorless (acidic form) to magenta (basic form) and has a pKa of 9.40. What is the ratio of the magenta phenolphthalein concentration to the colorless phenolphthalein concentration ([magenta phenolphthalein]/[colorless phenolphthalein]) at a pH of 11?

Answer 1

40:1 is the ratio of the magenta phenolphthalein concentration to the colorless phenolphthalein concentration.

Step-by-step explanation:

To calculate the pH of acidic buffer, we use the equation given by Henderson Hasselbalch:

`pH=pK_a+\log(([salt])/([acid]))`

`pH=pK_a+\log(([magenta(Php)])/([Php]))`

We are given:

`pK_a` = negative logarithm of acid dissociation constant of phenolphthalein = 9.40

`[magenta(Php)]` = concentration of magenta phenolphthalein

`[Php]` = concentration of colorless phenolphthalein

pH = 11

Putting values in above equation, we get:

`11=9.40+\log(([magenta(Php)])/([Php]))`

`\log(([magenta(Php)])/([Php]))=11-9.40=1.6`

`([magenta(Php)])/([Php])=10^(1.6)=39.81 :1 \approx 40:1`

40:1 is the ratio of the magenta phenolphthalein concentration to the colorless phenolphthalein concentration.