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Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: hope35 on February 16, 2010, 07:19:19 PM

Title: Determine concentration of bromine
Post by: hope35 on February 16, 2010, 07:19:19 PM

Hi, this is a question from my physical chemistry lab. So I have to determine the concentration of my original bromine solution. It given me ε for Br₂ at 400 nm is 160 L/mole *cm.

For my experiment, I diluted the orginal bromine solution. In a 100 mL volumetric flask, I used 8mL bromine solution and fill the rest with water. I took a UV vis spectrum of it and found a wavelength at 393.2nm.

Not very sure how I should approach this question.

Title: Re: Determine concentration of bromine
Post by: renge ishyo on February 17, 2010, 02:27:56 PM

You have to read the absorbance from your graph at 400 nm. Then together with the given ε, you can calculate the concentration in your diluted Br₂ sample using Beer's Law: A=εlc. Then you have to back calculate to the original concentration of the Br₂ before you diluted it to 100 ml (use M₁V₁=M₂V₂).

Title: Re: Determine concentration of bromine
Post by: hope35 on February 17, 2010, 03:32:21 PM

That looks right, but I don't know what "l" is. It was not given in the lab manual. Unless I'm suppose to look it up in the internet.

Title: Re: Determine concentration of bromine
Post by: renge ishyo on February 17, 2010, 03:37:34 PM

"l" is the path length of the cuvette you have used. It is usually 1 cm.