Question

You are asked to determine the amount of salicylic acid expressed in molar concentration ([Sal]) in an unknown sample of aspirin. You decide to use an UV-Vis spectrometer absorption method and run the following standards of Sal to construct a calibration curve.

1. Use the Excel program and prepare the plot Absorbance vs. [Sal].

2. Determine the equation for the best least squares fit (calibration equation).

3. Determine the concentration of salicylic acid in mol/L in your unknown sample of aspirin.

   [Sal], mol/L	Absorbance
   2.32 * 10^-4	0.059
   5.80 x 10^-4	0.147
   8.12 x 10^-4	0.234
   1.116 x 10^-3	0.274
   2.32 x 10^-3	0.568
   3.51 x 10^-3	0.853
   Unknown	0.371

Answer 1

Answer:

The plot [Sal] vs absorbance is shown below

From the plot, the straight line equation is

Absorbance=(239.25989)[Sal] + 0.01396 with best for R^2=0.997.

Given absorbance of unknown sample=0.371 then [Sal]=?

Therefore

0.371=(239.25989)[Sal] + 0.01396

[Sal]=(0.371-0.01396)/(239.25989)

[Sal]=1.492 x 10^-3 mol/L.

The concentration of unknown salicylic acid is 1.49x10^-3 mol/L.

Please let me know if you have any doubt. Thanks and I hope you like it.