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### Topic: Spectrophotometry  (Read 4152 times)

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#### thenewbosco

• Guest
##### Spectrophotometry
« on: March 04, 2005, 11:20:08 AM »
This is a question for a lab i have to do next week. I have never seen anything to do with this before. The theory given in the manual doesnt really help. The only thing it says is that absorption is proportional to concentration. Maybe you can give me some assistance on how to get started and some equations that i can use.

The eqilibrium constant for: AuBr4- + 2Au + 2Br- --> 3AuBr2- can be determined by preparing a solution of AuBr4- and AuBr2- in contact with a piece of gold metal.
In one experiment a solution initially containing 6.41x10-4 mol/L of dissolved gold (both AuBr4- and AuBr2- ) in 0.4 M HBr was allowed to attain equilibrium in the presence of gold metal. The absorbance was 0.445 in a 1.00 cm cell at 382nm.

In a separate experiment, the absorbance of a 8.54x10-5M solution of only AuBr4- (no AuBr2- ) in 0.4 M HBr was determined in a 1.00cm cell to be 0.410 at 382nm. AuBr2- does not absorb at 382nm.

a) calculate the equilibrium concentrations and b) evaluate the equilibrium constant.

I am pretty sure i can get b after i have a) but how do i go about getting the concentrations, any help is greatly appriciated.

Thanks
« Last Edit: March 04, 2005, 11:21:05 AM by thenewbosco »

#### savoy7

• Guest
##### Re:Spectrophotometry
« Reply #1 on: March 04, 2005, 05:57:21 PM »
Did you do a standard curve?  Do you have a regression formula (y=mx + b)?

Is this using Beer's Law?

savoy

#### savoy7

• Guest
##### Re:Spectrophotometry
« Reply #2 on: March 04, 2005, 06:17:14 PM »
If you didn't do a std curve - maybe you can use molar absorptivity.

Cell length you gave would be used.  Just do a search for molar absorptivity on the internet for the equation, I don't know it off hand.