GC #190 DAT Destroyer

[Muggs]

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Indicator pKa Protonated Color Deprotonated Color
A 4.4 Yellow Blue
B 5.0 Pink Orange
C 7.0 Yellow Purple
D 8.0 Blue Red

An aqueous solution has a hydroxide concentration of 1 x 10^-5 M would show what colors?

a. Yellow with indicator A
b. Blue with indicator B
c. Yellow with indicator C
d. Red with indicator D
e. Two of the above

Solution says to first find pH (pOH is 5, so pH is 9). Indicator A is blue at pH = 9.
this leads to my first question; so that means that protonated color refers to a solution with an acidic pH and deprotonated refers to a basic pH?

It goes on to say that indicator A will be blue at pH 9 since its pKa value is 4.4, so it will be deprotonated at this high pH.
I'm confused by this statement. Other than the fact that a low pKa value means a stronger acid, I don't see why a pKa of 4.4 means that the solution would be deprotonated at a pH of 9. Can someone explain this relationship to me clearer?

 

[EpicDentist]

Solution says to first find pH (pOH is 5, so pH is 9). Indicator A is blue at pH = 9.
this leads to my first question; so that means that protonated color refers to a solution with an acidic pH and deprotonated refers to a basic pH?

Yes, protinated=acidic pH and deprotinated=basic pH. Think of it like this, if the solution is protinated, then there must be lots of H+ in the pot, and therefore more H+ would make the pH acidic.

 

[chax1]

General Rule:

pH<pKa----> the compound is protonated

pH>pKa-----> the compound is deprotonated

I use the c-terminus of an amino acid as a reference in case I forget the rule. The C-terminus has a pka around 2.3 (give or take depending on the amino acid). Unless you have a really acidic pH, the C-terminus will almost always have a negative charge, COO- (i.e. deprotonated) because the pH is above the pKa in most scenarios. In the few scenarios where you are looking at pH<pKa... the C-terminus is neutral and is protonated (structure -COOH)....... I really hope this didnt confuse you more. Let me know if you need anymore clarification.

Chad explains this concept pretty well in his Organic video about proteins... I'm sure he may mention it somewhere in the Acid/Base set as well, i just can't recall where.

 

[Muggs]

General Rule:

pH<pKa----> the compound is protonated

pH>pKa-----> the compound is deprotonated

I use the c-terminus of an amino acid as a reference in case I forget the rule. The C-terminus has a pka around 2.3 (give or take depending on the amino acid). Unless you have a really acidic pH, the C-terminus will almost always have a negative charge, COO- (i.e. deprotonated) because the pH is above the pKa in most scenarios. In the few scenarios where you are looking at pH<pKa... the C-terminus is neutral and is protonated (structure -COOH)....... I really hope this didnt confuse you more. Let me know if you need anymore clarification.

Chad explains this concept pretty well in his Organic video about proteins... I'm sure he may mention it somewhere in the Acid/Base set as well, i just can't recall where.

that was perfect. thank you very much.