Question

Methyl red is a common acid-base indicator. it has a ka equal to 6.3 × 10–6. its un-ionized form is red and its anionic form is yellow. what color would a methyl red solution have at ph = 7.8?

Answer 1

Hello!

The pKa of an indicator tells us the pH where it changes color. The pKa of Methyl red is 

$pKa=-log(Ka)=-log( 6,3*10^{-6} )=5,20$

So, at a pH=7,8, Methyl Red would be at a pH over its pKa, so it would be in its basic form. The Basic Form of Methyl Red is Yellow, so the solution would be Yellow.

Have a nice day!