about double salt, FeSO4•(NH4)

1.
Double salt - salts contain two or moretypes of cation/anion, which are of regular crystalline arrangement.
E.g. ammoniumiron(II) sulphate (Mohr's salt)
** Note: whendouble salt is dissolved in water, it ionize in the same way that its componentionize separately
**Note:double salts' properties are not the same as the separate salts
e.g. Mohr'ssalt - when ionize, it give Fe 2+, NH4 + and SO4 2- which is as same asionization of FeSO4 and (NH4)2SO4 ; besides, its properties are not same asthat of FeSO4 and (NH4)2SO4

Complex ion - it has a metal ion at its centrewith a number of other molecules/ions(ligands), surrounding it. E.g. [Zn(OH)4]2- , [Cu(EDTA)]2- , etc.

The ligands can be considered bonding to the central ion byco-ordinate(dative) bonds.
(In some cases, the bonding is morecomplicated)

Difference - Asmentioned above, double salts ionize in the same way that its component ionizeseparately; but for complex ions, being in water, they still have a complexstructure, i.e. no change2.Because1) 4Fe 2+ +O2 + (2n+4) H2O <---> 2Fe2O3‧nH2O + 8H+When we areusing Mohr's salt, NH4+ will give out extensive protons, the above equilibrium willshift left. So, it remains at Fe2+ and is more stable.2) Mohr'ssalt's structure: H4N-O-(SO2)-O-Fe-O-(SO2)-O-NH4The valenceelectron of Fe is more likely to lose to the nearby O-atoms than FeSO4 does.So, there is less chance for the Fe in Mohr's salt to lose electron. Thus, itis more stable.Therefore,concentration of solution will be more accurate and there will be less error.Thus, Mohr'ssalt is preferred.
3.I GUESS it is to prevent oxidation in air.The reactivity depends on pH.Fe 2+oxidize in air rapidly only at high pH.So, sulphuric acid can prevents aerial oxidation.

圖片參考：http://imgcld.yimg.com/8/n/HA00656648/o/701201310119013873433440.jpg