So in the question , ferrous ammonium sulphate is Mohr Salt which is a double salt which in aqueous medium changes ferrous to ferric ions with loss of an electron


Equivalent weight = Molecular mass / change in valency

Since change is valency is one so equivalent weight of the salt will be equal to it's molecular weight !!!

Equivalent Weight of Ferrous Ammonium Sulphate

Ferrous ammonium sulfate, also known as Mohr's salt, is a double salt that contains both ferrous sulfate and ammonium sulfate. Its chemical formula is FeSO4(NH4)2SO4·6H2O. The equivalent weight of a compound is the amount of the compound that can react with or displace one equivalent of hydrogen or other positive element or radical. The equivalent weight of ferrous ammonium sulfate or Mohr's salt can be calculated using the formula:

Equivalent weight = Molecular weight / n

where n is the number of equivalents of the positive element or radical that the compound can form or displace. In the case of ferrous ammonium sulfate, the positive element is iron (Fe2+) and its equivalent weight can be calculated as follows:

Molecular weight of FeSO4(NH4)2SO4·6H2O = 392.14 g/mol
Number of equivalents of Fe2+ = 1
n = 1

Therefore, the equivalent weight of ferrous ammonium sulfate or Mohr's salt is:

Equivalent weight = 392.14 g/mol / 1 = 392.14 g/mol

Explanation

Ferrous ammonium sulfate or Mohr's salt is a common laboratory reagent that is used as a primary standard for the calibration of analytical instruments like spectrophotometers. It is also used in the preparation of other iron compounds like iron oxide and iron nanoparticles. The compound is a double salt that contains both ferrous sulfate and ammonium sulfate in its crystal lattice. The ferrous ion (Fe2+) is the active ingredient in the compound, and it can form one equivalent of positive charge or displace one equivalent of another positive element or radical in a chemical reaction. The equivalent weight of ferrous ammonium sulfate is therefore the molecular weight of the compound divided by the number of equivalents of Fe2+ that it can form or displace. The equivalent weight is an important parameter in analytical chemistry because it allows scientists to accurately measure the amount of a substance present in a sample or solution.