What is the solubility of Al(OH)3 at 25oC in 1.0M NaF solution? The Ksp for Al(OH)3 is 3.0x10-34, and the Kf for AlF63- is 4.0x1019, all at 25oC.
Solubility is most likely quite low, so we can simplify the problem in several ways.
1. We can assume concentration of F
- doesn't change and is 1M all the time (that will have to be checked once the calculations are done).
2. We can assume all dissolved Al
3+ is complexed, so the concentration of AlF
63- equals solubility (that will have to be checked as well).
3. If 2 is right, there is a simple dependence between concentrations of OH
- and AlF
63- (that will again require checking, as it may happen solubility is so low presence of OH
- from water autodissociation plays an important role).
At this moment you have three equations - Ksp, Kf, and the one mentioned in 3, and two unknowns - [Al
3+] and [AlF
63-] - simply solve for [AlF
63-], as per 2 that's the solubility. Then check if all assumptions hold.
Copper problem is slightly different, as presence of ammonia means already high concentration of OH
-.