1000 L of air at STP was dissolved in water and required 2.5  ×  10−5  moles of KMnO4 for complete reaction of SO2 as pollutants. Thus, SO2 content in air is

SO2 + H2O →  H2SO32MnO4−+7 + 5H2SO3+4 →   5H2SO4+6 + 2Mn2++22  moles  MnO4− =5  moles  H2SO3  or  SO22.5  ×  10−5  moles  MnO4− = 52  ×  2.5  ×  10−5  moles  SO2=6.25  ×  10−5  moles  SO2At STP,  6.25  ×  10−5  moles =6.25  ×  10−5 ×  22.4  L  SO2=1.4  ×  10−3  LIn  1000  L air,  SO2 = 1.4 × 10−3  LIn  106  L  ppm,  SO2 =  1.4 × 10−3 × 106103 =1.4  ppm