How does fluoride prevent tooth decay

Tooth enamel, the hard, protective outer layer of the tooth, is 98% hydroxyapatite (Ca5(P04)30H). Although quite insoluble K p = 6.8 x 10 in water), demineralization, which is the dissolving of hydroxyapatite, does occur especially when the saliva contains acids. The reverse reaction, remineralization, also occurs. Remineralization is the re-depositing of tooth enamel. When hydroxyapatite is in solution with fluoride ions, a double-replacement reaction can occur. Fluoride ion replaces the hydroxide ion to form fluoroapatite (Ca5(P04)3p, /Cjp = 1 X 10 °). Fluoroapatite remineralizes the tooth enamel, thus partially displacing hydroxyapatite. Because fluoroapatite is less soluble than hydroxyapatite, destructive demineralization is reduced. 

Write the equation for the dissolving of hydroxyapatite and its equilibrium constant expression. How do the conditions in the mouth differ from those of a true equilibrium  

Write the equation for the double-replacement reaction that occurs between hydroxyapatite and sodium fluoride. 

Calculate the solubility of hydroxyapatite and fluoroapatite in water. Compare the solubilities. 

What is the ion product constant (Ojp) for the reaction if 0.00050M NaF is mixed with an equal volume of 0.000015M Ca5(P04)30H Will a precipitate form (re-mineralization)  

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