Preparation of 0.25 N Ferrous Ammonium Sulfate (FAS) Solution and Stoichiometric Relationship with Sulfuric Acid (H2SO4)

Introduction: This guide will walk you through the process of preparing a 1 liter (1000 mL) solution of 0.25 N (normality) Ferrous Ammonium Sulfate (FAS) and the corresponding molarity of sulfuric acid (H2SO4).

Understanding Normality and Molarity

For Ferrous Ammonium Sulfate (Fe(NH4)2(SO4)2·6H2O, often simply represented as FAS), the relationship between normality (N) and molarity (M) is crucial in chemical preparations. Normality is a measure of concentration that is defined based on the number of equivalents of a substance per liter of solution, while molarity is based on the number of moles of solute per liter of solution. For FAS, one mole of the compound can donate one mole of Fe2 ions during an acid-base reaction, making the normality and molarity values equivalent.

Given that 1 mole of FAS provides 1 mole of Fe2 ions, a 0.25 N FAS solution is also 0.25 M. This relationship simplifies the preparation process and ensures accurate stoichiometry in reactions involving FAS.

Step 1: Calculating the Amount of FAS Required

To prepare 1 L of 0.25 N FAS, the first step is to determine the amount of FAS needed. Here’s how to do it:

Calculate the molar mass of FAS (Fe(NH4)2(SO4)2·6H2O): Fe: 55.85 g/mol N: 14.01 g/mol × 2 28.02 g/mol H: 1.01 g/mol × 28 28.28 g/mol S: 32.07 g/mol × 2 64.14 g/mol O: 16.00 g/mol × 8 128.00 g/mol Total molar mass: 55.85 28.02 28.28 64.14 128.00 278.10 g/mol

Calculate the mass of FAS required for 1 L of 0.25 M solution:

Mass Molarity × Volume L × Molar Mass

Mass 0.25 mol/L × 1 L × 278.10 g/mol 69.525 g

Step 2: Preparing the FAS Solution

Weigh out approximately 69.5 g of FAS.

Dissolve the FAS in a small volume of distilled water, less than 1 L.

Transfer the solution to a 1 L volumetric flask or a graduated cylinder.

Add distilled water to bring the final volume up to 1 L.

Step 3: Stoichiometric Relationship with Sulfuric Acid (H2SO4)

If you are using sulfuric acid (H2SO4) as a titrant or in a reaction involving FAS, it is essential to understand the stoichiometry of the reaction. FAS typically reacts with sulfuric acid in a 1:1 ratio during redox reactions. Therefore, to prepare a solution of H2SO4 that is also 0.25 N, you would prepare a solution that is 0.25 M, maintaining the 1:1 stoichiometric relationship.

Summary

To prepare 1 L of 0.25 N FAS, weigh out approximately 69.5 g of ferrous ammonium sulfate hexahydrate and dissolve it in water. For reactions involving this FAS solution, use 0.25 M H2SO4 as it will also be 0.25 N due to the 1:1 stoichiometry.

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