Question
From the solubility product constants for CaF2 (fluorite) and CaSO4 (anhydrite) determine the equilibrium constant for the reaction
CaF2 + SO4 2- --> CaSO4 + 2F-
CaF2 + SO4 2- --> CaSO4 + 2F-
Answers
eqn 1 CaF2 ==> Ca^2+ + 2F^-
eqn 2 Ca^2+ + SO4^2- ==> CaSO4
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sum 1+2 CaF2 + SO4^2- ==> (F^-)^2 + CaSO4
since CaF2 and CaSO4 are solids we can cancel them since they are not involved in the Keq. You note that eqn 1 is Ksp for CaF2 and eqn 2 is 1/Ksp for CaSO4; therefore, Keq for the reaction is
KspCaF2/KspCaSO4 = ?
eqn 2 Ca^2+ + SO4^2- ==> CaSO4
-------------------------------
sum 1+2 CaF2 + SO4^2- ==> (F^-)^2 + CaSO4
since CaF2 and CaSO4 are solids we can cancel them since they are not involved in the Keq. You note that eqn 1 is Ksp for CaF2 and eqn 2 is 1/Ksp for CaSO4; therefore, Keq for the reaction is
KspCaF2/KspCaSO4 = ?
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