Question

Ferric alum has the composition ${(N{H_4})_2}S{O_4}.F{e_2}{(S{O_4})_3}.x{H_2}O$ . Find the value of x.
A. 7
B. 24
C. 6
D. 15

Answer 1

Hint: Ferric alum is a double salt in the class of alums which consists of compounds with the general molecular formula $AB{(S{O_4})_2}.12{H_2}O$. Alums can have very complex structures; sometimes their structures even look like the structure of coordination compounds.

Complete Step by step answer: Ferric alum is also known as Ammonium (III) sulphate or ferric ammonium sulphate or iron alum. It is a double salt in the class of alums which consists of compounds with the general formula $AB{(S{O_4})_2}.12{H_2}O$ . It has the appearance of weekly violet, octahedral crystals. There has been some discussion regarding the origin of the crystals colour with some ascribing it to impurities in the compound and others claiming it to be a property of the crystal itself.
It is paramagnetic, acidic and toxic towards microorganisms. It is a weak oxidizing agent, capable of being reduced to Mohr’s salt which is ferrous ammonium sulphate.
It can be prepared by crystallization from a solution of ferric sulphate and ammonium sulphate. Iron in ferrous sulphate is oxidized to iron in ferric sulphate when sulphuric acid and nitric acid are added. Upon addition of ammonium sulphate to the solution and damping in the solution, ferric ammonium sulphate crystals precipitate. The nitric and sulphuric acid is reacted with the ferrous sulphate to make ferric sulphate, nitric acid and water. Then, ferric sulphate is mixed with ammonium sulphate and crystallized to get ammonium iron sulphate. The solution is normally tested to ensure that no more iron is left.
There are various uses of this alum. Some of these include waste water treatment, tanning, and production of dyestuffs and as an etching agent in the production of electronic components. Other applications include adiabatic refrigeration equipment, biochemical analysis and organic synthesis.
Now, according to the general formula we have 2 molecules of $S{O_4}$ and 12 molecules of water but according to the question we have 4 molecules of $S{O_4}$ . Therefore, we will have 24 molecules of water. So, the value of x is 24.

Therefore, the correct answer is option B.

Note: Ferric alum is also known as Ammonium (III) sulphate or ferric ammonium sulphate or iron alum. It is a double salt in the class of alums which consists of compounds with the general formula $AB{(S{O_4})_2}.12{H_2}O$ and formula ${(N{H_4})_2}S{O_4}.F{e_2}{(S{O_4})_3}.24{H_2}O$ .