Question

Calculate the minimum concentration of Mg2+ that mustbe added to 0.10 M NaF in order to initiate a precipitate ofmagnesium fluoride. (For MgF2, Ksp = 6.9 x10-9.)A. 1.4 x 107M
B. 6.9 x 10-9M
C. 6.9x 10-8 MD. 1.7 x 10-7M
E. 6.9 x 10-7 M

Answer 1

Answer:

E. 6.9 E-7 M

Explanation:

• NaF  →  Na+   +     F-

      0.10M   0.10M    0.10M

• MgF2 ↔ Mg2+  +    2F-

eq:     S             S          2S + 0.1

∴ Ksp = 6.9 E-9 = [Mg2+][F-]² = (S)(2S+0.1)²

if we compared the concentration ( 0.1 M ) with the Ksp ( 6.9 E-9 ); then we can neglect the solubility as adding:

⇒ Ksp = 6.9 E-9 = (S)(0.1)² = 0.01S

⇒ S = 6.9 E-9 / 0.01 = 6.9 E-7 M