Cu+ reacts with neocuproine to form the coloured complex (neocuproine)2Cu+, with an absorption maximum at 454 nm. Neocuproine is particularly useful because it reacts with few other metals. The copper complex is soluble in 3-methyl-1-butanol, an organic solvent that does not dissolve appreciably in water. If (neocuproine)2Cu+ is present, virtually of it goes into the organic phase. For the purpose of this problem, assume that all of the coloured complex transfers into the organic phase. Supposed that the following procedure is carried out:
A rock containing copper is pulverized, and all metals are extracted from it using strong acid. The acidic solution is neutralized with base and made up to 250.0 mL in flask A.
Next, 15.00 mL of the solution is transferred to flask B and treated with 15.00 mL of reducing agent to convert Cu2+ to Cu+. Then 15.00 mL of buffer are added so that the pH is suitable for complex formation with neocuproine.
25.00 mL of this solution are withdrawn and placed in flask C. To the flask are added 15.00 mL of an aqueous solution containing neocuproine and 30.00 mL of 3-methyl-1-butanol. After the mixture has been shaken well and the phases allowed to separate, all (neocuproine)2Cu+ is in the organic phase.
A few milliliters of the organic layer are withdrawn, and the absorbance at 454 nm is measured in a 1.00-cm cell. A blank carried through the same procedure gives an absorbance of 0.072.
a. Suppose that the rock contained 3.00 mg of Cu. What will be the concentration of Cu (mol/L) in the organic phase?
b. If the molar absorptivity of (neocuproine)2Cu+ is 7.90 x 103 M-1 cm-1, what will be the observed absorbance?
c. Another rock is analyzed in the same manner and found to give a final absorbance of 0.753 (uncorrected for the blank). How many milligrams of Cu are in the rock?
If I read this right, you have 3 mg Cu initially, you place it in 250 mL, withdraw 15, add 15 more of something else and take 25 of that 30. So you should have 0.003/63.55 mol = ? initially and at the end it will be
(0.003/63.55) x (15/250) x (25/30) = ? and that is in 30 mL of the solvent so ?/0.030L = approx 8E-5M for part a.
A = e*b*c
A = 7.90E3*1.00*about 8E-5M will give you the A with no blank. Add 0.072 to that for the reading for part b. I think that is about 0.632 and add 0.072 for the final of about 0.704. But note that the 8E-5 is an estimated value so the numbers won't be exactly as above.
c. A uncorrd = 0.753
.............-0.072
--------------------
A corrected = 0.753-0.072 = 0.681
Here is what I would do for part c. I would forget all of those dilution steps and extraction steps and recognize that everything done to the standard was done to the sample, also. Then I would compute a new "corrected" molar absorptivity(call it k). It won't be the molar absorptivity but it will be k when c is in mg and it will work the problem.
A = k*b*c
0.681 = e*b*3 mg Cu
k = 0.632/3 = 0.210 and use that for the new sample.
A = k*b*c
0.681 = 0.210*1*c
c = about 3.24 mg (still estimated).
You can do it another way.
3 mg Cu x (0.681/0.632) = about 3.24 mg.
a. To find the concentration of Cu in the organic phase, we need to determine the number of moles of Cu that transferred to the organic phase.
Given:
- The rock contained 3.00 mg of Cu.
- The volume of solution in flask A is 250.0 mL.
- 25.00 mL of the solution from flask B is transferred to flask C.
First, let's convert the mass of Cu to moles:
1 mole of Cu = 63.546 g
3.00 mg of Cu = 3.00 x 10^-3 g of Cu
Number of moles of Cu = (3.00 x 10^-3 g) / (63.546 g/mol) = 4.72 x 10^-5 mol
Since 25.00 mL of the solution from flask B is transferred to flask C, and the volume in flask A is 250.0 mL, we can calculate the dilution factor:
Dilution factor = (25.00 mL) / (250.0 mL) = 0.100
To find the concentration in the organic phase, we can multiply the concentration in flask B by the dilution factor:
Concentration of Cu in the organic phase = (4.72 x 10^-5 mol) x (0.100) = 4.72 x 10^-6 mol/L
Therefore, the concentration of Cu in the organic phase is 4.72 x 10^-6 mol/L.
b. To calculate the observed absorbance, we can use Beer-Lambert Law:
Absorbance = molar absorptivity x path length x concentration
Given:
- The molar absorptivity of (neocuproine)2Cu+ is 7.90 x 10^3 M^-1 cm^-1.
- The path length is 1.00 cm.
We already calculated the concentration of Cu in the organic phase to be 4.72 x 10^-6 mol/L.
Now we can plug in the values:
Absorbance = (7.90 x 10^3 M^-1 cm^-1) x (1.00 cm) x (4.72 x 10^-6 mol/L)
Absorbance = 3.73 x 10^-5
Therefore, the observed absorbance is 3.73 x 10^-5.
c. For this part, we can use the observed absorbance (0.753) and the molar absorptivity (7.90 x 10^3 M^-1 cm^-1) to calculate the concentration of Cu in the organic phase:
0.753 = (7.90 x 10^3 M^-1 cm^-1) x (1.00 cm) x (Cu concentration)
Rearranging the equation, we get:
Cu concentration = 0.753 / [(7.90 x 10^3 M^-1 cm^-1) x (1.00 cm)]
Cu concentration = 9.53 x 10^-5 mol/L
Now, we want to find the mass of Cu in the rock. We can calculate the number of moles of Cu in flasks A and B (including the dilution factor) and use the ratio to find the mass:
Number of moles of Cu in flask A and B = (9.53 x 10^-5 mol/L) x (0.100) = 9.53 x 10^-6 mol
Mass of Cu in the rock = (9.53 x 10^-6 mol) x (63.546 g/mol)
Mass of Cu in the rock = 0.605 mg
Therefore, the number of milligrams of Cu in the rock is 0.605 mg.
To answer these questions, we need to use the given information and follow the steps described in the problem.
a. First, we need to determine the concentration of Cu in the organic phase. We know that all of the (neocuproine)2Cu+ complex transfers into the organic phase.
We are given:
- 15.00 mL of the solution is transferred to flask B.
- The solution in flask B contains Cu2+, which is converted to Cu+.
- 25.00 mL of this solution is withdrawn and placed in flask C.
- The (neocuproine)2Cu+ complex is formed in flask C and transferred to the organic phase.
From the information given, we can calculate the moles of Cu in the organic phase as follows:
1. Calculate the moles of Cu in the initial solution in flask B:
- Determine the moles of Cu2+ using the balanced equation for the reduction of Cu2+ to Cu+.
- Convert the moles of Cu2+ to moles of Cu using the molar ratio.
2. Calculate the concentration of Cu in the initial solution in flask C:
- Divide the moles of Cu by the volume in liters.
3. Calculate the concentration of Cu in the organic phase:
- Since all of the (neocuproine)2Cu+ complex transfers into the organic phase, the concentration of Cu in the organic phase will be the same as the concentration in the initial solution in flask C.
b. To determine the observed absorbance, we need to use the molar absorptivity, concentration, and path length (cell thickness) of the sample.
The formula to calculate absorbance is:
Absorbance = molar absorptivity * concentration * path length
We are given:
- The molar absorptivity of (neocuproine)2Cu+ is 7.90 x 103 M-1 cm-1.
- The path length is 1.00 cm.
Using the given information and the concentration of Cu in the organic phase calculated in part a, we can calculate the observed absorbance.
c. To determine the number of milligrams of Cu in the rock, we need to use the observed absorbance and the equation used in part b:
Absorbance = molar absorptivity * concentration * path length
We are given:
- The observed absorbance is 0.753 (uncorrected for the blank).
Using the given information and the observed absorbance, we can rearrange the equation and solve for the concentration of Cu.
Finally, we can convert the concentration of Cu to the number of moles, and then to the mass of Cu using the molar mass of Cu.