The complex ion, [Fe(phen)_3]^+2, has a deep-red color in solution. The absorban
ID: 967329 • Letter: T
Question
The complex ion, [Fe(phen)_3]^+2, has a deep-red color in solution. The absorbances for several standard solutions containing [Fe(phen)_3]^+2 were measured at 510 nm in a Spectronic 20 Genesys using a cuvette with a 1.0 cm path length. The data is shown below. What is the value of the molar absorptivity, epsilon, for [Fe(phen)_3]^+2 at 510 nm? A stock solution of zinc nitrate (Zn(NO_3)_2) was prepared by placing 0.91 g of solid (Zn(NO_3)_2) in a 25.00-mL volumetric flask and diluting to the mark with water. A standard solution was then prepared by transferring 2.5 mL of the stock solution to a second 25.00-mL volumetric flask and diluting to the mark. What is the concentration (in M) of (Zn(NO_3)_2) in the stock solution? What is the concentration (in M) of (Zn(NO_3)_2) in the standard solution?Explanation / Answer
According to Beer's Law,
Absorption, A = cl
from standard solution 1,
= A/cl = 0.983/(8.985*10-5 M)(1 cm)
= 10940 M-1.cm-1
from standard solution 2,
= A/cl = 0.747/(6.757*10-5 M)(1 cm)
= 11055 M-1.cm-1
from standard solution 3,
= A/cl = 0.607/(5.427*10-5 M)(1 cm)
= 11185 M-1.cm-1
from standard solution 4,
= A/cl = 0.502/(4.492*10-5 M)(1 cm)
= 11175 M-1.cm-1
Mean value of = 10940+11055+11185+11175/4 = 11089 M-1.cm-1
2. molar mass of Zn(NO3)2 = 65.41 + 2*14 + 6*16 = 189.41 g/mol
a. 0.91 g = 0.91/189.41 = 0.0048 moles
concentration of stock solution
= 0.0048 moles * 1000/25 mL
= 0.192 M
b. V1S1 = V2S2
where, V1 = initial volume
S1 = initial concentration
V2 = total final volume
S2 = final concentration
concentration of standard solution
= V1S1 / V2 = 2.5 mL * 0.192 M/25 mL = 0.0192 M
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