You weigh out a piece of copper wire (AW=63.546): Weight of copper standard.....

Question

You weigh out a piece of copper wire (AW=63.546):
Weight of copper standard....................0.2437 g
Dissolve it in a slight excess of concentrated nitric acid. Add to the solution the required amount of concentrated ammonia. Transfer this quantitatively to a 100 mL volumetric flask, dilute to volume, and mix thoroughly.
10.00 mL of this is pipetted into a 100 mL volumetric flask, the required amount of concentrated ammonia is added and diluted to volume. This is Standard 1.
Standard 2 is made by pipetting 20 mL of the original solution into a 100 mL volumetric flask, adding the required amount of ammonia, NH3, and diluting to volume.
Using the same cuvet for all measurements, the following %T's were obtained at 625 nm:
Blank 101.3 %
Standard 1 61.0 %
Standard 2 35.5 %

Next:
You pipetted 10 mL of UNKNOWN into a 100 mL volumetric flask. The required amount of ammonia was added and the mixture diluted to volume. The %T of this solution was measured at 625 nm, using the same spectrometer and cuvet as for the standards.
%T unknown.................... 21.4 %
CALCULATE
a) The absorbance of Standard 1 ______________
b) The absorbance of Standard 2 ______________
c) The molarity of Cu in Standard 1 ______________ M
d) The molarity of Cu in Standard 2 ______________ M
e) Ratio: absorbance/molarity Std 1 ______________
f) Ratio: absorbance/molarity Std 2 ______________
g) Average ratio of absorbance/molarity ______________
h) Absorbance of unknown solution ______________
Concentration of copper in unknown...
i) solution as measured in cuvet ______________ M
j) in original solution ______________ M
k) in original solution ______________ w/V%

Finally:
You are given a solid UNKNOWN, containing copper, weighing........ 0.7937 g.
You dissolve this in concentrated Nitric Acid, HNO3, transfer quantitatively to a 100 mL volumetric flask, add the required amount of Ammonia, NH3, dilute to volume and mix thoroughly.
This resulting solution is too concentrated and the resulting %T measured in the same cuvet and spectrometer as before gave little or no transmission (<10%). br="">You are told to pipet.................... 16.00 mL
into a 100 mL volumetric flask, add the required amount of ammonia, dilute to volume and mix.
Spectroscopic measurement of this solution gave %T... 35.6 %
CALCULATE:
l) Absorbance of the diluted solution....................._____________________
m) The copper concentration in the diluted solution...._____________________ M
n) The copper concentration in the original solution..._____________________ M
o) The Per Cent copper in the original solid UNKNOWN..._____________________ % Cu

Explanation / Answer

a)% Transmittance of standard 1 = 61.0%

Transmittance = 0.61

Absorbance = 0.39

b)%T =  35.5 %

Transmittance = 0.355

Absorbance = 1- 0.355 = 0.645

c)Mass of copper = 0.2437 g

It is diluted to 100 mL

10 mL of this solution is taken and diluted to 100 mL

So mass of copper present = 0.2437 / 10 = 0.02437 g

Molarity = 0.02437 g * 1000 / (63.546 g/mol * 100 mL)

           = 0.0038 M

d) For standard 2, 20 mL of the inital solution is taken and diluted to 100 mL

So the molarity will be double i.e. = 2 * 0.0038 M = 0.0076 M

e) Absorbance: molarity = 0.39 / 0.0038

                                = 102.6 :1

f) Absorbance: molarity = 0.645 / 0.0076

                                 = 84.8 : 1

g) Average ratio: average of 102.6 and 84.8

Absorbance : molarity = 93.73:1

h)

%T = 21.4 %

T = 0.214

Absorbance = 0.786

Using the average ratio: Molarity = Absorbance / 93.73

                                              = 0.786 / 93.73

                                             = 0.0084 M

i)

The calculated concentration is that present in the cuvet

j) In original solution, the concentration will be 10 times more since 10 mL of solution was diluted to 100 mL

So concentration = 0.084 M

Hope this helps

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