- #1

RoboNerd

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## Homework Statement

Methyl red has the following structure [not listed here on this post]

Ka = 5.0 * 10 E -5

It undergoes a color change from red to yellow as a solution gets more basic.

- Calculate an approximate pH range for which the methyl red is useful.
- What is the color change and the pH at the color change when a weak acid is titrated with a strong base using methyl red as an indicator?
- For which of these two types of titrations is methyl red a possible indicator?

## Homework Equations

Henderson Hasselbalch equation

## The Attempt at a Solution

OK. I tried to figure this problem out, but I have no idea if I am right or not.

If anybody could check, that would be great. Thanks.

For the approximate pH range:

- For the pH range, I took the pH range to be within range of pKa plus/minus 1. So did: -log(5.0 * 10E-6) and then I added and subtracted 1.
- I got the answer to be [4.30, 6.30]

- Color changes from red to yellow. Then, the pH at the color change will be [second bullet]
- pH = pKa + log( [In-] / [HIn] ) =
**-log (5.0 E -6) + log ( 1/10 ) =**4.30

- Color change: yellow to red
- pH? I did the same thing as in the previous step only my log expression had
**log(10/1)**. I got my pH to be 6.30.

For which of these two types of titrations is methyl red a possible indicator?

The methyl red is a possible indicator for the weak base and strong acid titration.

Any input on whether my solutions are right or not are greatly appreciated.

Thanks in advance!