# Ka of weak acid indicator

## Homework Statement

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.

GCT
Homework Helper
l.o.g.

Borek
Mentor
Do you know what definition of pKa is?

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

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

Borek
Mentor
That's the way you do it 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?

Borek
Mentor
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?

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

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.

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

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?

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

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.

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

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

Potassium hydroxide (KOH)
Caesium hydroxide (CsOH)
Sodium hydroxide (NaOH)
Lithium hydroxide (LiOH)
Rubidium hydroxide (RbOH)

Second easy group:

Strontium hydroxide (Sr(OH)2)
Calcium hydroxide (Ca(OH)2)
Barium hydroxide (Ba(OH)2)

Third easy group:

Hydroiodic acid HI
Hydrobromic acid HBr
Hydrochloric acid HCl

And only four left:

Perchloric acid HClO4
Sulfuric acid H2SO4 (Ka1/first dissociation only)
Nitric acid HNO3
Hydronium ion H3O+ or H+

Can you tell how they were grouped?

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.

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

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