Question

III. Interpretation of Results (a) Calibration curve Calculate the Cu2+ concentration of each standard solution in ug/mL (ug = 100 gram). Alternatively, ug/mL = ppm (part per million). Plot the net absorbance (= absorbance of standard - absorbance of blank) versus the concentration of each standard solution.

the molar absorptivity of the copper complex in chloroform at 436.0nm is 13,000 M^-1 cm^-1 which is useful for the determination of Cu²⁺ concentrations as low as 10^-6g/ml.

Answer 1

ANSWER:

Part (a) - Calibration curve

As you proide the molar absorptivity of Cu+2 we can calculate the concentration of Cu+2 in each standard solution using the Beer's law:

A= Exxo

where A = absorbance, E = molar absortivity, l = path length of cell and c = concentration of Cu+2 solution

Assuming a path length of 1 cm, then,

C = А 13000 M-1 εχι 13000 M-1cm-1 x 1 cm

example with sample 1

c= 0.061 13000 M-1 - = 4.69x10-6 M

Volume	Absorbance	[Cu+2] (mol/L)
0	0.061	4.69E-06
5	0.095	7.31E-06
10	0.125	9.62E-06
20	0.201	1.55E-05
30	0.260	2.00E-05
40	0.360	2.77E-05

Now, we transform concentrations of Cu+2 from mol/L to ug/mL:

+2 1 6 3.55 g ug/mL = [Cu+?) 1* 1000 mL^ 1 mol 100 g 19

example of calculation:

jug/mL = 4.96.c10-6 mo mol L 1L 63.559 1000 mL^ 1 mol 100 ug - 0.298 M9 19 mL

then

Volume	Absorbance	[Cu+2] (mol/L)	Cu+2 (ug/mL)
0	0.061	4.69E-06	0.298
5	0.095	7.31E-06	0.464
10	0.125	9.62E-06	0.611
20	0.201	1.55E-05	0.983
30	0.260	2.00E-05	1.271
40	0.360	2.77E-05	1.760

Now we can made the plot of Absorbance (y-axis) vs. Cu+2 (x-axis):

Absorbance vs. Cu*2 (ug/mL) y = 0.2046x - 1E-16 R2= 1. Absorbance 0.000 0.500 1.500 2.000 1.000 Cu2 (ug/mL)

and the equation is

$y=0.2046x-8.33x10^{-17}$

or

$\mathbf{A=0.2046\times Cu^{+2}-8.33x10^{-17}}$

NOTE: instead of using Beer's law to calculate the concentration of coper standards, a better way to calculate the concentrations of standards (Cstd) is using the following equation:

Cstd =- Cstock Vstock Vstd

where Cstock = concentration of concentrated solution of Cu+2 used to prepare the standard solutions, Vstock = aliquote of concentrated solution of Cu+2 used to prepare the standard solution, Vstd = volume of standard solution prepared

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Part (b) - Determination of copper

Using the equation obtained from the grpah in the calibration curve we can calculate the concentration of copper in the unknown solutions:

A = 0.2046 x Cu+2 – 8.33.c10-17

$Cu^{+2}=\frac{A+8.33x10^{-17}}{0.2046}$

then, the concentration of copper in the unknown solutions are

	Absorbance	Cu+2 (ug/mL)
unknown 1	0.206	1.007
unknown 2	0.306	1.496
unknown 3	0.206	1.007

And the concentration of copper in the original solution is

Cutie = Cunk * 10.00 mL 500 ml

then

Sample	Cu+2 unknown(ug/mL)	Cu+2 original (ug/mL)
1	1.007	50.351
2	1.496	74.793
3	1.007	50.351