A series of standard solutions containing a red dye was made by diluting a stock
ID: 898670 • Letter: A
Question
A series of standard solutions containing a red dye was made by diluting a stock solution and then measuring the percent transmittance of each solution at 505 nm. The results, after conversion to absorbance, are shown below.
An absorbance of 0.47was also determined at 505 nm for a solution with an unknown concentration of the red dye.
solution
concentration
absorbance
blank
0.00 M
0.00
standard #1
0.15 M
0.24
standard #2
0.30 M
0.50
standard #3
0.45 M
0.72
standard #4
0.60 M
0.99
sample
???? M
0.47
The absorbance of the blank and standards were plotted as a function of their concentration. The line of best fit has an equation of y=1.76x-0.005.
A) The percent transmittance of each red dye solution was measured at 505 nm, which is greenish blue. Why?
B) What is the concentration of the unknown dye with an absorbance of 0.47?
solution
concentration
absorbance
blank
0.00 M
0.00
standard #1
0.15 M
0.24
standard #2
0.30 M
0.50
standard #3
0.45 M
0.72
standard #4
0.60 M
0.99
sample
???? M
0.47
Explanation / Answer
A) The absorbance readings are always taken at the lambda max which is at 505 nm for red dye. This is the point of maximum absorbance for the dye.
B) concentration of the unknown can be found from the equation,
y = 1.76x - 0.005
For the unknown,
y = 0.47
find x
0.47 = 1.76x - 0.005
x = 0.27 M
So the concentration of unknown would be 0.27 M
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