# Homework Help: Ka estimate

1. Nov 4, 2008

### magma_saber

1. The problem statement, all variables and given/known data
Estimate the Ka the using the assumption that the color change can be seen by our eyes when [ln-]/[Hln] = 1/10
to see the acid form or [ln-]/[Hln] = 10/1 to see the color associated with the base.
Use these numbers with your measured [H+] to calculate a Ka for each indicator you measured.

2. Relevant equations
acid-base base-acid H+
Phenolphthalein 8.7 7.4 8.91E-09

I got the hydronium concentration by taking taking the average of the ph values and doing 10^-ph. How do i get the ka estimates?

Do i set up the equation like this: ka = 1/[H+] or something like this?

2. Nov 4, 2008

### Staff: Mentor

Describe the experiment that you did. I am almost sure you have not posted all information that you have.

Last edited: Nov 4, 2008
3. Nov 4, 2008

4. Nov 4, 2008

5. Nov 4, 2008

### magma_saber

ya i know that the phs are wrong. The bromophenol blue was really off. When calculating the H+ concentration, i only used 4.2, not 9.3.