G chem question, HELP!!!!!!!

[DDSelin2mori]

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Can anyone please help me with this problem from Destroyer:

An indicator is red n the undissociated form and yellow in the dissociated form. If a solution were at a very low PH, what color would the indicator appear?
a) Red b) Yellow c) Orange d)purple e) Unable to tell, more information needed

I would say the answer is E since we don't know if the indicator is a acis or base but it is answered as A assuming that most indicators are organic acid. Shouldn't we choose the best answer? and we don't know for sure if this is an acid or base.

Thanks

 

[bigstix808]

Can anyone please help me with this problem from Destroyer:

An indicator is red n the undissociated form and yellow in the dissociated form. If a solution were at a very low PH, what color would the indicator appear?
a) Red b) Yellow c) Orange d)purple e) Unable to tell, more information needed

I would say the answer is E since we don't know if the indicator is a acis or base but it is answered as A assuming that most indicators are organic acid. Shouldn't we choose the best answer? and we don't know for sure if this is an acid or base.

Thanks

i would say the key statement in this question is "very low pH". since you are correct in that most indicators are organic acids you can almost rest assured that in a very acidic solution that indicator is going to be undissociated and would henseforth make it A) RED.

 

[tooth123]

firstly i wouldn't write out the entire question due to copyright issues.
secondly i think this is a good example of how everyone trashes kaplan, but i still think it is a very valuable source.
page 937 in the kaplan book

undissociated dissociated
HIN ----> H+ + IN-
color1 color 2
red yellow

very low pH means its really acidic, so we have a lot of H+ according to Le Chatelier lots of H+ shifts the equilibrium to the opposite side, to the left therefore the solution will be red.
hope this helps!

 

[Danny289]

Can anyone please help me with this problem from Destroyer:

An indicator is red n the undissociated form and yellow in the dissociated form. If a solution were at a very low PH, what color would the indicator appear?
a) Red b) Yellow c) Orange d)purple e) Unable to tell, more information needed

I would say the answer is E since we don't know if the indicator is a acis or base but it is answered as A assuming that most indicators are organic acid. Shouldn't we choose the best answer? and we don't know for sure if this is an acid or base.

Thanks

i

indicators are usually weak acids or bases, which when dissolved in water dissociate slightly and form ions. for indicators there is term "KIn is known as the indicator dissociation constant" and just remember for acidic (weak acid)
indicator KIn=[H+] and for basic indicator it will be kIn=[OH-] (In= indicator)
just remeber Weak acid or weak base!

assume this indicator is weak acid: it is clear it can not be dissociated in strong acid and the collor will be red.
now assume this is weak Base:
for basic indicators there is not going to be any [OH-] in very acidic solution then no changeing color.
this is my opinion maybe there will be better explanation.

 

[Orgodox]

The indicator is a weak acid and is red in the undissociated form. Therefore at a low pH (ie acidic) the equilibrium will shift to the left towards the undissociated form which is red!

 

[DDSelin2mori]

Thanks guys. I didn’t notice that if the indicator is even a weak base it will be in its acidic form in that PH and still undissociated so the color will be Red.

 

[DDSguyLA]

ONLY transition Metals create color, keep that in mind.

 

[Orgodox]

That is true but irrelevant here. We are discussing indicators. The whole point of an indicator IS TO CHANGE COLORS when reaching a certain pH