A 0.050 M solution of Zn(NO₃)₂ is at 25 °C. The solubility-product constant of Zn(OH)₂ at this temperature is Ksp=3.0×10−17. What is the pH at which Zn(OH)₂(s) just begins to precipitate when small amounts of NaOH(aq) are added?
a) 5.1
b) 6.9
c) 6.4
d) 8.9