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The unknown containing element Z was mixed with aliquots of a standard solution of Z as shown in the table. When analyzed, the mixtures gave the absorbance readings listed. When plotted, the graph to the right resulted. The equation for the line is y = (8.00*10^-5)x+0.162. The concentration of Z in the unknown is

Note: I have attached a pdf picture of the graph and the table since I don't know how to include it here.

A) 8*10^-5 micrograms/mL
B) 1.6 micrograms/mL
C) 200 micrograms/mL
D) 2000 micrograms/mL

I wasn't quite sure how to do this problem, and I still don't really know, which is why I'm asking. The problem never gave me a single absorbance, which I could then perhaps use to calculate the concentration as I have done this before. But then I was unsure which point to use, and so I thought maybe it had to do with slope, but A is wrong, so can anyone guide me to the right answer please?

[Borek]

How is the slope of the curve related to concentration?

How is the intercept of the curve related to concentration?

[Arkcon]

Look closely at your table.  What absorbance do you get for nothing + unknown?  What absorbance do you get for 4000 ug + the same amount of unknown?  This will give you a hint as to what multiple guess answer it might be, or definitely isn't.  This will help you visualize the answer in the way you're accustomed to -- where a single point standard gives an OD value, and you take an unknown OD value and calculate backward.  However, to do this problem correcting in a non-multiple guess, you will have to learn how to use slope and intercept, like Borek: explained.  The class probably wouldn't just dump you in the middle like this, you should have had some explanation of what to do.

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Thanks, Arkcon. I suppose I should've thought of that since this was multiple choice, although I will still attempt to learn how to do it were this not a multiple-choice question.

How is the slope of the curve related to concentration?

How is the intercept of the curve related to concentration?

Is it that the intercept can give you the concentration of the unknown solution? And the slope is how much absorbance changes as the concentration of the standard increases, so you can figure out the concentration of the standard somehow?