Note 350 — Chapter IX · SYSTEMATIC ANALYSIS FOR THE COMMON METAL IONS. THE IONS OF THE ALKALIES AND OF THE ALKALINE EARTHS. ORDER OF PRECIPITATION OF DIFFICULTLY SOLUBLE SALTS WITH A COMMON ION
Reading edition
Putting [HO−] = [NH4+] = y, we have y2 : (0.2 − y) = 18E−6. Then y = 0.0019.