Beer\'s Law A Beer\'s Law graph obtained by plotting absorbance versus concentra

Question

Beer's Law

A Beer's Law graph obtained by plotting absorbance versus concentration in mol/L for a series of standard solutions, produced a straight line with equation
y = 2815.1 x

Suppose an unknown SCN solution is treated in exactly the same manner as described in Part B of the procedure.  
The final diluted solution gives an absorbance reading of 0.235.  
Determine the [FeSCN2+] in this diluted solution.

8.4×10-5 M

Determine the SCN concentration in the original (undiluted) unknown solution.

Attached is Part B

PART B: Determining ISCN-l in an Unknown Solution In Part B, you work independently. Furthermore, you are not permitted to use the automatic pipette. You may only use the 10.00 mL volumetric pipette to deliver both the Fe" and thiocyanate (SCN) solutions. Since the 10.00 mL tric pipette was used for dispensing the Fe solution in Part A, make sure to rinse the pipette thoroughly with water before starting the Part B procedure. Note that there is a second 10.00 mL volumetric pipette in your drawer that will be used by the other individual in your pair to do the Part B procedure. You are provided with a solution of unknown SCN- concentration. Use your general knowledge of lab technique (gained in exp. 2) to guide you through these steps. Transfer 10.00 mL of the unknown into a 100.00 mL volumetric flask and dilute the solution to the mark with 0.1 M HNO3. Stopper and invert the flask 8-10 times to mix well. Pipette 10.00 ml of this diluted solution into a 25.00 mL volumetric flask. Add 10.00 mL of 0.150 M Fes solution and dilute to the mark with 0.1 M HNo,. Mix well. Rinse and fill the cuvette with this solution and read the absorbance value displayed. Do NOT click on collect. Record the displayed absorbance value in your notebook. Do not remove your unknown until you have completed the printing as outlined below.

Explanation / Answer

Given that plotting absorbance versus concentration in mol/L for a series of standard solutions, produced a straight line with equation
y = 2815.1 x where y = absorbance and x = concentration

Given that absorbance = 0.235

   [FeSCN2+] = ?

Then,

y = 2815.1 x

0.235 = 2815.1  [FeSCN2+]

[FeSCN2+] = 0.235 / 2815.1

= 8.4×10-5 M

Therefore,

[FeSCN2+] = 8.4×10-5 M

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