By balancing redox reaction:
10[FeSO4.(NH4)2SO4.6H2O]+2KMNO4+8H2SO4→5Fe2(SO4)3+2MnSO4+10(NH4)2SO4+K2SO4+68H2O
8H++MnO4−+5Fe+2⟶5Fe+3+Mn+2+4H2O
For 2 moles of MnO4−,8moles of H2O. are released 5 moles of Mohr's salt contain 30 moles of H2O. So, 10 moles of Mohr's salt contain 60 moles of H2O So, total moles of H2O produced=68