2H3AsO4+5H2S→Conc.HClAs2S5+8H2O
2 moles of Arsenic Acid → 1 mole of Arsenic Pentasulphide
1 moles of Arsenic Acid → 1/2 mole of Arsenic Pentasulphide
MolarmassofH3AsO4=141;MolarmassofAs2S5=308
∴ Number of moles of H3AsO4=14135.5=0.25
∴ Number of moles of As2S5=20.25=0.125mol