Fluorite into a 0.352 M aqueous soi or this probletm (a) Set up the equilibrium
ID: 637009 • Letter: F
Question
Fluorite into a 0.352 M aqueous soi or this probletm (a) Set up the equilibrium variable to be solved by the geologist's finding, decided to try dissolving a lump of (b) Insert the variable used in the eqilbrium table into the Ksp expression (e) Calcu solution. (Use the Newton-Raphson procedure if necessary). late the equilibrium concentration of Ca? and F ions within the chemist's (d) Did the geologist's pond or the chemist's solution have a higher concentration of calcium ion? (e) Name and briefly describe the principle that explains this difference in calcium ion concentration.Explanation / Answer
a) Fluorite contains calcium fluoride, CaF2. CaF2 is sparingly soluble in water and the solubility if given as
CaF2 (s) <======> Ca2+ (aq) + 2 F- (aq); Ksp = [Ca2+][F-]2 = 3.45*10-11
Sodium fluoride, NaF ionizes completely in aqueous solution as below.
NaF (aq) -------> Na+ (aq) + F- (aq)
Since 1 mole NaF = 1 mole F-, therefore,
[F-] = 0.352 M.
Note that [F-] from ionization of NaF is much higher than [F-] obtained from CaF2 because CaF2 is sparingly soluble in water. Therefore, we can approximate the total [F-] in the aqueous solution of NaF containing fluorite to be 0.352 M.
Thus, we set up the equilibrium table as below.
CaF2 (s) <=======> Ca2+ (aq) + 2 F- (aq)
initial -
equilibrium x 0.352
Let the concentration of Ca2+ in solution = solubility of CaF2 be x M. We are required to determine the numerical value of x.
b) Write down the Ksp expression for the dissolution equation as
Ksp = [Ca2+][F-]2 = (x).(0.352)2
c) Now solve the above expression.
Ksp = (x).(0.123904)
=====> 3.45*10-11 = (x).(0.123904)
=====> x = (3.45*10-11)/(0.123904) = 2.7844*10-10 ? 2.78*10-10
The equilibrium concentrations of Ca2+ and F- ions in the chemist’s solution are 2.78*10-10 M and 0.352 M respectively. Note that we have started by saying that the concentration of F- obtained from the ionization of NaF is much higher than that obtained from CaF2 (ans).
d – e) Need to know the value obtained by the geologist.