Question

Problem: You weigh out a piece of copper wire (AW=63.546): Weight of copper standard....................0.2992 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 101.1 %

Standard 1 58.8 %

Standard 2 33.0 %

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.................... 49.2 %

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........ 1.4198 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%). You are told to pipet.................... 3.00 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... 49.2 %

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

Answer 1

Abs Std1=-log(0.581)=0.2306

Abs Std2=-log(0.33)=0.481

The first solution was the 0.2992 g of Cu in 100 mL

the Std 1 was prepared with 10 mL of the previous solution to a volume of 100mL

you can use the equation C1V1=C2V2

The Std 2 was prepared with 20 mL of the 0.0471M solution

Ratio is just the quotient of Abs over M

Average ratio 50.01 M^-1

Abs of the unknown= 0.308

The concentration is calcualted by dividing the Abs over the ratio

<-----this is i)

the concentration in the original solution can be calculated as above, since you diluted 10 mL(V2) into 100mL(V1).

---->%(m/V)

Second part

Abs=0.308

copper in the dilute solution

<----m

The dilution was 3 mL in 100mL:

<----n

the percent is calculated using the volume from the solution(100mL or 0.1L) and the weight of the sample:

<--- 77.33%Cu