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

Answer:

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

Explanation:

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

$pH=pK_a+\log(\frac{[salt]}{[acid]})$

$pH=pK_a+\log(\frac{[magenta(Php)]}{[Php]})$

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:

$11=9.40+\log(\frac{[magenta(Php)]}{[Php]})$

$\log(\frac{[magenta(Php)]}{[Php]})=11-9.40=1.6$

$\frac{[magenta(Php)]}{[Php]}=10^{1.6}=39.81 :1 \approx 40:1$

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