Question

What is the ratio of the magenta phenolphthalein concentration to the colorless phenolphthalein concentration ([magenta phenolphthalein]/[colorless phenolphthalein]) at a pH of 10.9?

Answer 1

Answer:

$\frac{[magenta\ phenolphthalein]}{[colorless\ phenolphthalein]}=31.62$

Explanation:

Considering the Henderson- Hasselbalch equation for the calculation of the pH of the buffer solution as:

$pH=pKa+\log\frac{[base]}{[acid]}$

Where Ka is the dissociation constant of the acid.

pKa of phenolphthalein = 9.40

pH = 10.9

So,

$10.9=9.40+\log\frac{[magenta\ phenolphthalein]}{[colorless\ phenolphthalein]}$

$\frac{[magenta\ phenolphthalein]}{[colorless\ phenolphthalein]}=31.62$