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Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: kimxx972 on June 28, 2009, 01:34:19 AM

Title: GRE chemistry subject question
Post by: kimxx972 on June 28, 2009, 01:34:19 AM

Cresol red indicator has two color changes in the pH range 0-14.
pH range       acid color      base color
0.2-1.8            red              yellow
7.2-8.8           yellow             red

What colors are to be expected in solutions at pH values of 1.0, 6.0 and 9.0?

                1.0              6.0               9.0
(A)            Red            Red              Yellow
(B)            Red            Yellow          Yellow
(C)            Orange       Yellow          Red
(D)            Yellow        Orange         Red
(E)            Red            Red              Orange

The ans is (C).

Can anybody explaing why the answer is C ?

Thank you

Title: Re: GRE chemistry subject question
Post by: nj_bartel on June 28, 2009, 02:27:32 AM

pH 1 is between 0.2 and 1.8.  You blend red and yellow to get orange.  pH 6 is between 1.8 and 7.2.  You're in the yellow range of both indicators.  pH 9 is above 8.8 and is thus in the red range.

Title: Re: GRE chemistry subject question
Post by: Borek on June 28, 2009, 03:41:29 AM

More quantitative discussion:

http://www.titrations.info/acid-base-titration-indicators

Title: Re: GRE chemistry subject question
Post by: kimxx972 on June 28, 2009, 11:10:47 AM

Thanks for your simple but understandable explanation.

The website is also very helpful to catch up the definition of titration.

:)

Title: Re: GRE chemistry subject question
Post by: hey chikitha on December 03, 2009, 01:18:01 AM

Hey guys,
When will be the results out of nov 7 chem gre...
I am freaking out.
chem..

Title: Re: GRE chemistry subject question
Post by: jag300 on December 18, 2009, 11:45:57 PM

Quote from: hey chikitha on December 03, 2009, 01:18:01 AM

Hey guys,
When will be the results out of nov 7 chem gre...
I am freaking out.
chem..

U should have your scores by now