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.