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

Question

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 41.1 mg sample of brass, and the resulting diluted solution was found to contain 15.52 mg/L of Cu, what is the %Cu in the sample?

Explanation / Answer

15.52 mg/L of Cu = (0.01552/63.59) mol/L = 2.44*10-4 M

No of moles of Cu in the final 100 mL solution

= 2.44*10-4 * 100/1000 = 2.44*10-5 moles

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

= 2.44*10-5 * 1000/5

= 4.88*10-3 M

Amount of Cu in the concentrated 100 mL solution

= 4.88*10-3 * 100/1000 = 4.88*10-4 moles

= 4.88*10-4 moles * 63.59 g/mol

= 30.10 mg

% Cu in the sample

= (30.10/41.7) *100

= 74.58%

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