ANAL 359 Pre-Laboratory Assignment (2) Convert percent transmittances to equiva-
ID: 704592 • Letter: A
Question
ANAL 359 Pre-Laboratory Assignment (2) Convert percent transmittances to equiva- lent absorbances, using Equation 4 1. Read an authoritative source for of 2. The two components of the solution you will be care. List these two substances and describe the spectrophotometry a discussion ofe (2) Convert percent transmitances to equiva- 180.5 2to4,71% problems associated with each (3) Prepare a Beer's law plot for (Fe(phenanthroline)al2, using the data obtained in best straight line through the data 3. 1,10-Phenanthroline forms a bright red complex (1 and (2). Draw a with iron/Il). The complex has an analytical wave points ength of 510 nm and has a high molar absorptivity. Hydroxylamine hydrochloride is added to keep the ot iron(Il) was treated with iron in a reduced +2 state, and the pH is controlled us ing an acetate buffler. A series of iron(l)-phen- (4) A solution containing an unknown amount hydroxylamine hydrochlo- ride, de- scribed above. The percent transmittance of the sam- acetate buffer, and 1,10-phenanthroline as en anthroline solutions were made by adding various ple at ?-510 nm was 52.4 when read against a blank containing all but the iron(lI) solution. Determine the amounts of 4.300 x10-M Fe+ to 5 mL each of hydroxylamine hydrochloride, acetate buffer, 1,10-phenanthroline. Distilled water was added to make the total volume of each solution 50.00 mL. The percent transmittance of each solution was read, us- ing a blank containing everything but the iron(Il) solu- tion. Data for the determination are and molar concentration of iron(Il) in the unknown. (5) Calculate the molar absorptivity for of stock soliton,mL of stock solution,mL %T 100 80.35 7.00 21.78 2.00 64.71 9.00 14.09 5.00 33.65 10.00 11.32 %T (1) Calculate the molar concentration of [Fe(phenanthroline) 2. Assume a 1.00-cm path- [Fe(phenanthroline)32 in each solution. length. 1.00 860oExplanation / Answer
2) The two chemicals used in this lab are potassium permanganate (KMnO4) and sulfuric acid (H2SO4).
KMnO4 causes skin and eye irritation. In case KMnO4 comes in contact with skin, inflammation and blistering of the skin are common. KMnO4 can cause damage to the cornea of the eyes leading to blindness.
H2SO4 is a strong oxidizing acid. H2SO4 I strongly corrosive and causes skin and eye damage.
3) (1) The balanced chemical equation for the reaction is given as
Fe2+ + 3 phenanthroline ----------> [Fe(phenanthroline)3]2+
As per the balanced stoichiometric equation,
1 mole Fe2+ = 1 mole [Fe(phenanthroline)32+]
Therefore,
molarity of Fe2+ = molarity of [Fe(phenanthroline)3]2+ (since the volume stays the same).
Determine the molarity of Fe2+ in solution 1 using the dilution equation. The dilution equation is
C1*V1 = C2*V2
where C1 = 4.300*10-4 M (concentration of stock Fe2+); V1 = 1.00 mL = volume of stock Fe2+ taken to prepare solution 1; V2 = final volume of the solution = 50.00 mL and C2 = concentration of Fe2+ in the dilute solution = concentration of [Fe(phenanthroline)32+] in the solution. Plug in values and obtain
(4.300*10-4 M)*(1.00 mL) = C2*(50.00 mL)
====> C2 = (4.300*10-4 M)*(1.00 mL)/(50.00 mL) = 8.600*10-4 M.
Fill in the table as below.
Volume of stock solution, mL
Concentration of Fe2+ = concentration of [Fe(phenanthroline)32+], M
1.00
8.600*10-6
2.00
1.720*10-5
5.00
4.300*10-5
7.00
6.020*10-5
9.00
7.740*10-5
10.00
8.600*10-5
(2) The absorbance (A) and the percent transmittance (%T) of a solution are related as
A = 2 – log (%T)
Taker solution 1 as an example. We have %T = 80.35; therefore,
A = 2 – log (80.35) = 0.0950
Fill in the table below.
Volume of stock solution, mL
%T
A = 2 – log (%T)
1.00
80.35
0.0950
2.00
64.71
0.1890
5.00
33.65
0.4730
7.00
21.78
0.6619
9.00
14.09
0.8511
10.00
11.32
0.9461
(3) Prepare a plot of A (y-axis) vs concentration of [Fe(phenanthroline)32+] (x-axis as below).
Plot of absorbance vs concentration of standard [Fe(phenanthroline)32+]
(4) The percent transmittance of the unknown sample of iron (II) is 52.4, i.e, %T = 52.4.
Therefore,
A = 2 – log (%T) = 2 – log (52.4) = 0.2807
Use the regression equation in the plot above and put y = 0.2807 and find out x which is the concentration of iron (II) in the unknown sample. Therefore,
0.2807 = 10996x + 0.0001
=====> 10996x = 0.2807 – 0.0001 = 0.2806
=====> x = 0.2806/10996 = 2.5518*10-5
The concentration of iron (II) in the unknown sample is 2.5518*10-5 M (x has unit M) (ans).
(5) We know that, as per Beer’s law,
A = ?*C*l
where ? = molar absorptivity of [Fe(phenanthroline)32+], C = concentration of [Fe(phenanthroline)32+] in M and l = 1.00 cm is the path length of the solution. Therefore,
? = A/Cl …..(1)
Again, as per the regression equation,
y (absorbance) = slope*(concentration) + constant …..(2)
Comparing (1) and (2), we have,
? = slope = 10996 M-1.cm-1 (ans).
Note the unit of ?. A is dimensionless; C has unit M and l has unit cm. Therefore, ? must have unit M-1.cm-1.
Volume of stock solution, mL
Concentration of Fe2+ = concentration of [Fe(phenanthroline)32+], M
1.00
8.600*10-6
2.00
1.720*10-5
5.00
4.300*10-5
7.00
6.020*10-5
9.00
7.740*10-5
10.00
8.600*10-5
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