# Ka of weak acid indicator

1. Oct 26, 2008

### magma_saber

1. The problem statement, all variables and given/known data
How do i get the Ka of weak base indicators such as bromophenol blue, methly red, bromothymol blue, and phenolophalein? I found the pKa values online but i couldn't find thier Ka values.

2. Oct 26, 2008

### GCT

l.o.g.

3. Oct 26, 2008

### Staff: Mentor

Do you know what definition of pKa is?

Relation between pKa and Ka is identical to that between pH and H+[/sup concentration.

4. Oct 26, 2008

### magma_saber

i have the pH so for example the pH of one substance is 3.8, so i just do this: 10^-3.8?

5. Oct 26, 2008

### Staff: Mentor

That's the way you do it

6. Oct 26, 2008

### magma_saber

how do you know is an acid or base is strong? I know that strong acids and bases completely dissociates, but how would i know if it completely dissociates?

For weak acids you can find the pH by taking the negative log of the square root of Ka times the initial concentration of the acid. But if its a strong acid, you only have to take the negative log of the initial concentration.
i.e. 6 M HCl, the pH is -log(6.0). What is the pH of 3 M HNO3?

7. Oct 26, 2008

### Staff: Mentor

There not that many strong acids and bases, you have to remember them. Everything else is weak.

That's only approximation, one that works if some conditions are fulfilled. See discussion of weak acid pH calculation.

HNO3 is one of those strong acids you have to remember.

8. Oct 26, 2008

### magma_saber

I found these in wikipedia. I'm guessing that these are the only one's i'm gonna have to remember.

Potassium hydroxide (KOH)
Barium hydroxide (Ba(OH)2)
Caesium hydroxide (CsOH)
Sodium hydroxide (NaOH)
Strontium hydroxide (Sr(OH)2)
Calcium hydroxide (Ca(OH)2)
Lithium hydroxide (LiOH)
Rubidium hydroxide (RbOH)
Perchloric acid HClO4
Hydroiodic acid HI
Hydrobromic acid HBr
Hydrochloric acid HCl
Sulfuric acid H2SO4 (Ka1/first dissociation only)
Nitric acid HNO3
Hydronium ion H3O+ or H+

So other than these compounds, almost all the rest are all weak acids or bases right?
also the pH of 3 M of HNO3 is -0.48 right? and the pOH is 14.48? i only need the approximations.
Sorry for keep on asking you questions btw.

9. Oct 26, 2008

### Staff: Mentor

Yes, but note that they can be combined. One easy group:

Second easy group:

Third easy group:

And only four left:

Can you tell how they were grouped?

OK. Although for so highly concentrated solutions pH is no longer just -log of the concentration - but that's completely different story.

10. Oct 26, 2008

### magma_saber

thanks for all the help. i got an exam in three days. this really helped out.