Gen Chem question: Acid-Base

[neoba]

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The question says that there is an indicator which is red when undissociated and yellow when dissociated, and asks for the color of it when it is in a strong acidic environment.
My thought is that the indicator can be both a weak base or a weak acid. A weak acid will be undissociated at low pH, while a weak base will be dissociated. Thus whether the indicator is dissociated will depends on the nature, i.e. whether it's a base or an acid.
However, the answer says it should be red, hence undissociated, regardless whether it is base/acid.

Can anyone help?
Thanks a lot!

Just want to add another quick question regarding valence electron and quantum number： Which set of quantum numbers best describe a valence electron in Germanium?
The correct answer is 4,1,1,1/2. But shouldn't 4,0,0,1/2, which describe a 4s electron, also be a valid answer?

Thanks again!

 

[StanleyThomas]

You have most of the concepts in the indicator problem correct, except for assuming that the indicator is variable. The indicator exists in two forms based on the conditions of the environment, not on the conditions of the indicator. The indicator can exist as an acid-base pair and will be undissociated in strongly acidic environment (assumed to be below its pKa) and dissociated in environments above its pKa.


For your second question... yes 4,0,0,1/2 should be a valence electron of Germanium as well.

 

[neoba]

Thanks for your reply, StanleyThomas.

I am not quite sure what you were referring to when you say

You have most of the concepts in the indicator problem correct, except for assuming that the indicator is variable.

can you explain a little bit more?

Also, you said

The indicator ... will be undissociated in strongly acidic environment (assumed to be below its pKa) and dissociated in environments above its pKa.

I guess that is what confuses me the most. I thought that can only be applied to weak acids, but not weak bases? And we don't know whether the indicator is a base or acid? Am I making any sense?

 

[neoba]

Found this at http://chemistry.about.com/od/acidsbases/a/strengthacids.htm

Weak Bases

• Most weak bases are anions of weak acids.
• Weak bases do not furnish OH- ions by dissociation. Instead, they react with water to generate OH- ions.

I guess that explains it?

B- + H2O <===> HB + OH- and B- is the dissociated form. When there are excess H+, the equilibrium is pushed to the right and transform the base into the undissociated form.

 

[StanleyThomas]

Yes, except in strong acid H20 would exist primarily as H30+ or H+, thus Le Chatelier's principle would be easier to view.
So the formula to visualize would be: H30+ + B- <=====> HB + H20
In strong acid the indicator B-/HB, like you said, would exist in the undissociated HB form.

B-/HB is the indicator.
Indicators are almost always weak acid/base conjugate pairs.
All acids/bases exists as conjugate pairs. Always understand this when visualizing/thinking about acid/base problems.

Strong acid ---> Very weak conjugate base
Weak acid --> Very Strong conjugate base
Weak acid (somewhere in middle range of pKa) --> Weak base (somewhere in middle range of pKa) [ THESE are usually used as indicators]

 

[neoba]

Thanks!