122k views
2 votes
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?

User Evil
by
8.6k points

1 Answer

5 votes

Answer:

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.

User Heiko Oberdiek
by
8.2k points
Welcome to QAmmunity.org, where you can ask questions and receive answers from other members of our community.