absorbance at 525 nm. In the equation for the line, the y-variable represents the

absorbance, and the x-variable represents the concentration.

Measuring a Solution of Unknown Concentration

If we take and measure an unknown solution and find that it has an absorbance

of 1.5 at a wavelength of 525 nm, then we can use that information to determine

the concentration. One important point we should always keep in mind is that we

want the measurement of the unknown to fall in between our lowest and highest

measurements. In this example, an absorbance of 1.5 is higher than my highest

point on the calibration curve. Since this is the case, I would dilute the unknown

by taking say 10 mL of the unknown and diluting it to a new volume of 25 mL.

Now when I measured the absorbance of the diluted unknown, I get an

absorbance of 0.587. This value can be used with my calibration curve because it

falls within the measurements I have already taken. Now I can use the equation

for the line to calculate the concentration of the diluted unknown.

Rearranging to solve for “x” which represents the concentration of the diluted

unknown gives:

This gives that the concentration of the unknown sample is 0.000116 M. This

number makes sense since I could approximate it by finding the point on the

dotted line that has an absorbance of 0.587 and reading off the concentration.

In order to know the concentration of the original unknown, we must take into

account the dilution using

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