Question

A student prepares a series of solutions of caffeine of known
concentration and measures their absorbance values. A linear
regression analysis was conducted on the data from Experiment 5
using Excel and produced the following equation for the line:

Y = 0.4399 X + 0.125

The absorbance value for the student's unknown sample is 2.500
units. The student dilutes the unknown by placing 1.00 mL of the
unknown solution into a 25.0 mL volumetric flask and diluting to
the fill line with water. If the absorbance of the diluted sample
is 0.55, what is the concentration of the unknown in the
**original** (undiluted) sample?

Answer #1

The first thing to do is to match the absorbance value with the concentration, we have a value of 0.55 so:

Y = 0.4399 X + 0.125; (This equation is tipycall for absorbance, y is absorbance and x is concentration)

0.55 = 0.4399 X + 0.125

x = (0.55 - 0.125) / 0.4399

X = concentration = 0.966 M

So this is the concentration for a sample of 25 ml.

At the beginning we have just 1 ml with an unknown concentration of M, now we can apply the next relatioship

M1*V1 = M2*V2

so:

M1 * 1 ml = 0.966M * 25 ml

M1 = 0.966M * 25ml / 1ml = 24.15 M

since we have the same units on the same side we can make calculation with ml.

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