Question

Tooth enamel consists of hydroxyapatite, Ca5(PO4)3(OH) with a Ksp of 6.8x10-37. In water the has the potential break down using the reaction:

Ca5(PO4)3(OH) (s) « 5Ca2+(aq) + 3PO33-(aq) + OH-(aq)
Fluoride ions are added to the water to react with the Ca5(PO4)3(OH) and form the m ore tooth decay-resistant fluoride apatite, Ca5(PO4)3F (Ksp=1.0x10-60). Fluorinated water has dramatically decrease cavities in children, (a) calculate the solubility of Ca5(PO4)3(OH) in water and (b) will the Ca5(PO4)3F be more or less soluble?

Answer 1

- Cas (PO4)₂ OH = ksp = 6.8 x 1037 a) det solubility in water = smolar cas (PO4)3OH - 5 cat2 + 3 Pou3 + Ort i ksp 55 35 s Ksp = (carys (Poū3]} Comi] = (s )* (33)(s)' 6.8*1837 = 55x33 g s9 = 8:059 x 1042 Is =2.72 x 10sm are b) Cas (Poulzf , Ksp = 1x 1o 60 det solubility in water = smoar cas (Pou), OH 2 5 Cat? + 3 poj3 + f ; ksp 55 35s Ksp = (carr 5 (POū373 CF ] = (5 5)*(35)3 (s) 1x 1860 = 55. 33 xg9 59 = 1• 185 x 1065 s = 6:11x100 m ove so solubility is less than cas(PO4), OH Are