Hydroxyapatite, Ca 10 ( PO 4 ) 6 ( OH ) 2 , has a solubility constant of Ksp = 2.34 × 10 − 59 , and dissociates according to Ca 10 ( PO 4 ) 6 ( OH ) 2 ( s ) − ⇀ ↽ − 10 Ca 2 + ( aq ) + 6 PO 3 − 4 ( aq ) + 2 OH − ( aq ) Solid hydroxyapatite is dissolved in water to form a saturated solution. What is the concentration of Ca 2 + in this solution if [ OH − ] is fixed at 3.90 × 10 − 6 M ?