A standard solution of iron was made by weighing 0.075 g of Fe(NH4)2(SO4)2 6(H2O

Question

A standard solution of iron was made by weighing 0.075 g of Fe(NH4)2(SO4)2 6(H2O) in 250 mL. Aliquots of this standard solution (see below) were transferred to a 100 mL volumetric flask, pH adjusted with citrate, and reacted with hydroquinone and o-phenanthroline and diluted to volume (100 mL). The absorbance was measured in a 1.0-cm cell using a Genesys 20 Spectrophotometer at 508 nm:

A pill weighing 0.410 g was dissolved in 25 mL HCl and diluted to 100 mL. Of this solution, a 10.0 mL aliquot was transferred to a clean 100-mL volumetric flask and diluted to volume. Finally, a 10.0-mL aliquot of this dilute solution was added to a clean 100-mL volumetric flask with 6 mL citrate, 2.00 mL of hydroquinone, and 3.00 mL o-phenanthroline and diluted to the mark (100.0 mL). After standing for 10 minutes, the absorbance was read 3 times in a 1.0 cm cell, and the average absorbance was 0.226.

Use these data to report:

(1) the molar absorptivity of the iron o-phenanthroline complex at 508nm, and

(2) the mass of iron in the pill sample in mg.

Aliquot of standard solution Absorbance 1.0 mL 0.079 2.0 mL 0.163 5.0 mL 0.413 10.0 mL 0.842

Explanation / Answer

Standard solution

molar concentration of stock solution = 0.075 g/392.13 g/mol x 0.250 L = 7.65 x 10^-4 M

Aliquots diluted to 100 ml

Using M2 = M1V1/V2

with M2 is final concentration, M1 is initial stock solution concentration, V1 and V2 are initial and final solution volumes

Volume of stock (ml)      Final concentration (M)

                 1                           7.65 x 10^-6

                 2                           1.53 x 10^-5

                 5                           3.82 x 10^-5

                10                          7.65 x 10^-5

Plot Absorbance on y-axis and concentration on x-axis.

(1) slope = molar absorptivity of iron-phenanthroline complex = (0.842 - 0.079)/(7.65 x 10^-5 - 7.65 x 10^-6) = 11082 M-1.cm-1

(2) Unknown

Absorbance = 0.226

concentration of complex in final diluted solution = 0.226 x 7.65 x 10^-5/0.842 = 2.053 x 10^-5 M

concentration of complex in original 100 ml solution of pill = 2.053 x 10^-5 x 100 = 2.053 x 10^-3 M

mass of iron in the pill = 2.053 x 10^-3 M x 100 ml x 55.84 g/mol = 11.4 mg

Related Questions

Navigate

• Browse (All)
• Browse A
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.