For the following equilibrium
NH₃+H₂O⇔NH₄⁺ + OH−
calculate the equilibrium constant, if for the equilibrium,
NH₄⁺ + H₂O⇔NH₄OH + H⁺

the equilibrium constant is 5.5×10⁻¹⁰
A. 1.8×10⁻⁴
B. 1.8×10⁻⁵
C. 1.8×10⁻⁶
D. 1.8×10⁻⁷