1. You are using the following procedure from your lab instructions to determine

Question

1. You are using the following procedure from your lab instructions to determine the copper content of a brass sample: "Approximately 35 mg of a dried brass sample (accurately measured) is put into a 250-mL Erlenmeyer flask. About 20 mL of H2O and 10 mL of concentrated HNO3 is added. This sample is digested, then quantitatively transferred to a 100-mL volumetric flask and diluted to the mark. The sample is still too concentrated, so a 1:20 dilution is accomplished by transferring 5.00 mL of this solution to another 100-mL volumetric flask and diluting to the mark." If you actually used a 41.7 mg sample of brass, and the resulting diluted solution was found to contain 14.49 mg/L of Cu, what is the %Cu in the sample?

2)At a pH of 4, the Cu-neocuproine complex formed in the liquid-liquid extraction experiment has a partition coefficient of about 6.6 when equilibrated between the acid aqueous solution and chloroform. Calculate the concentration of Cu in the chloroform phase if 35.0 mL of a 0.20 M aqueous Cu solution buffered to pH 4 is equilibrated with 12.0 mL of chloroform

Explanation / Answer

1.

14.49 mg/L of Cu = (0.01449 /63.59) mol/L = 2.28*10-4 M

No of moles of Cu in the final 100 mL solution

= 2.28*10-4 * 100/1000 = 2.28*10-5 moles

This amount of Cu came from the concentrated 100 mL solution. Concentration of this solution

= 2.28*10-5 * 1000/5

= 4.56*10-3 M

Amount of Cu in the concentrated 100 mL solution

= 4.56*10-3 * 100/1000 = 4.56*10-4 moles

= 4.56*10-4 moles * 63.59 g/mol

= 28.98 mg

% Cu in the sample

= (28.98/41.7) *100

= 69.49 %

2.

Amount of Cu = 0.2*35/1000 = 0.007 moles

Fraction of solute(Cu) remaining in the aqueous phase after extraction can be expressed as:

faq = Vaq/(K*Vorg + Vaq)      [where, Vaq = volume of aqueous solution; K = partition coefficient; Vorg = volume of organic solution]

= 35/(6.6*12 + 35)

= 0.3

The fraction of solute in the organic phase is (1 – 0.3) or 0.7

So 70% of the solute will be transferred to the organic phase.

concentration of Cu in the chloroform

=[ (0.007 * 70% ) moles * 1000/12] M

= 0.408 M

3.

Absorbance, A = cl [from Lambert Beer's law]

First determine = A/cl = 0.218/(6.104 * 10-4M )*(1 cm)

= 357.14 mol-1.L.cm-1

Concentration of Cu in the unknown solution

= A/l

= 0.428/[(357.14 mol-1.L.cm-1) * (2.50 cm)]

= 4.79*10-4 M

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