Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: yg7s7 on November 18, 2008, 10:52:00 AM
Title: Spectrophotometry calculation
Post by: yg7s7 on November 18, 2008, 10:52:00 AM
A solution was prepared by mixing 0.025 L of 0.0800 M aniline, 0.025 L of 0.0600 M sulfanilic acid, and 0.001 L of 1.23 * 10^-4 M HIn (protonated indicator) and then diluting to 0.1 L. The absorbance measured at 550 nm in a 5.00 cm was 0.110. Find the conc. of HIn and In- and pKa for HIn.
I have no idea how to do this. Any ideas? I set up equation 0.110 = [HIn]*5.00*(2.26*10^4) + [In-]*5.00*(1.5*10^4), but I'm stuck. How do you find the absorbance at wavelength = 325? The answer key says pKa = 4.00
Title: Re: Spectrophotometry calculation
Post by: ARGOS++ on November 18, 2008, 11:35:35 AM
Dear yg7s7;
You’re right: One equation with two unknowns is not solvable; but you may have forgotten that
[HIn] + [In-] = ctotal of Ind.
Don’t forget the dilution steps!
I hope I have been of some help to you. Good Luck! ARGOS++
Title: Re: Spectrophotometry calculation
Post by: yg7s7 on November 18, 2008, 02:24:44 PM
so you mean I have to do (1.23*10^-4) * 0.001 L / 0.1 L and use it as [HIn] + [In-] = 1.23*10^-6?
Title: Re: Spectrophotometry calculation
Post by: ARGOS++ on November 18, 2008, 05:53:29 PM
Dear yg7s7;
That’s exactly correct! And now have you any suggestion, how you find the ratio: [HIn] / [In-], and from that the individual concentration's of both?
Good Luck! ARGOS++
Title: Re: Spectrophotometry calculation
Post by: ARGOS++ on November 19, 2008, 02:34:42 PM
Dear yg7s7;
Could this additional hint guide you into the right direction?: What would you read on your spectrometer as A550nm: - if [HIn] = ctotal of Ind?; and - if [In-] = ctotal of Ind? And finally: What ratio [HIn] / [In-] must result if A550nm is equal 0.110?
May this be of any help to you? Good Luck! ARGOS++