Calculate the formal molar concentration of 1, 10-phenanthroline in your sample

Question

Calculate the formal molar concentration of 1, 10-phenanthroline in your sample preps (don't forget the dilution from the reagent prepared to the analyte solutions measured). The molecular mass is 181.2 g/mol. Calculate the highest formal molarity of iron (i.e. 2.5 ppm) measured. What is the theoretical stoichiometric molar ratio required for this reaction (no excess reactant) (see Method section)? What is the actual molar ratio of complexing agent-to-iron in your highest concentration solution? Although the formation constant for the iron-phenanthroline complex is high, to assure complete reaction, an excess of the complexing reagent should be supplied in the method. Is an excess of complexing reagent supplied for all standards and samples in this experiment? As shown in #2 above, all Fe^2+ exists as Fe(phen)_3^2+ after reaction. Thus the formal concentration of Fe^2+ is 2 5 ppm, but the equilibrium species concentration is [Fe^2+] = 0 ppm after reaction. What is the equilibrium molar concentration of the complex. (Fe(phen)_3^2+]? In the units of molarity, it is the same as the formal molar concentration of Fe. Using the molar equilibrium concentration of the non-phenanthroline complex for one of the standards or samples, assume the path length is 1.0 cm, and use Beer's law to calculate the molar absorptivity of the complex. Include the units in your answer.

Explanation / Answer

2. Molarity of 1,10-phenanthroline in solution = g/molar mass x volume

= 0.1/181.2 x 0.1

= 5.52 mmol

1 mole of iron reacts with 3 moles of 1,10-phenanthroline

So,

molarity of iron in complex = 3 x 5.52 = 16.56 mmol

Theoretical stoichiometric molar ratio required (iron : 1,10-phenanthroline) = 1 : 3 molar ratio

The excess complexing agent is added to ensure all of the iron in solution is complexed completely. Thus, the same is done in all the other solutions as well, to have all the iron in comlexed form before measurement so no iron dissolves from th complex once formed. Equilibrium lies on the right.

3. Equilibrium molar concentration of complex in solution

= 2.5 ppm

= 2.5 mg/L

= 2.5/596.475

= 0.0042 mM

the units here is mM as compared to M for iron.                        

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