a) Explain in bullet points how to prepare the dilution series. c) Calculate the

Question

a) Explain in bullet points how to prepare the dilution series.

c) Calculate the mass % of copper in the alloy.

d) If the cuvette used to measure the dissolved sample had finger smudges, how would the calculated mass % of copper be affected? Explain

f) Describe another lab technique that could be used to determine the mass % of Ag in the sample.

g) H+ + Ag(s) + HNO3 --> Ag+1 + NO2 + H2O ;

2H+ + Cu(s) + 2HNO3 --> Cu+2 + 2NO2 + 2H2O

The 2 equations are for the dissolution of Ag and Cu. What assumption could the student make about the sample to insure that excess 16 M HNO3 is used to dissolve the alloy sample?

Explanation / Answer

a) the first solution is the 0.1M, it is the stock solution. Then if you want to know how much volume of the stock you need to prepare another solution with different concentration use this formula:

Vstock x Cstock= Vdiluted x Cdiluted

So, for example in order to obtain the second solution (0.08M):

Vstock= 25mL x 0.08M/0.1M= 20mL

I assume that you need to prepare 25mL of each solution, because the volume needed to measure absorbance is small, 25 mL is more than enough.

You can make the solutions in to ways, preparing all from the stock solution or from the previous diluted solution.

Now bullet points for the method from the stock solution:

-take 20 mL of the stock solution with a volumetric pipette.

-put it into a 25mL volumetric flask and brought to volume with distilled water.

-Mix.

You have to do the math to the rest of the concentrations but the procedure is the same.

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With the values of the table you can make an equation: y=8x

y= absorbance

x=Molarity

Now, the value of absorbance of our sample is 0.23 ----> 0.23= 8 X ----> X= 0.02875M

This concentration is in the 100mL in wich we disolved the sample. So, the amount of moles of cooper is:

M= mol/V ----> mol= 0.02875M x 0.1L= 2.875x10-3 mol ---> mass= 2.875x10-3 mol x 63.546g/mol= 0.183g

%Cu= 0.183g/0.478g x 100= 38.22%

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d) The absorbance is measured by the amount of light that comes through the cuvette, so, if the cuvette is dirty, less light will come through the cuvette so the instrument will inform a higher concentration, because if less light come through the cuvette, the absorbance will be higher, and that means more concentration.

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f) you can dissolve the sample in HCl concentrated in order to obtain a white precipitate, AgCl. Then you can dry it and weigh it.

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g) sorry i don't have idea of what the teacher is trying to say

Let me know if you have any doubt

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