Whis is the relationship between DG and DGf° for the reaction

MgF2(s) ===> Mg2+(aq) + 2 F-(aq)

Is it:
DG = {DGf°[Mg2+(aq)] + 2 DGf°[F-(aq)] - DGf°[MgF2(s)]} + RT ln([Mg2+][F-]2)

or

DG = {DGf°[Mg2+(aq)] + 2 DGf°[F-(aq)] + RT ln([Mg2+][F-]2)

or am I completely wrong?

if youre ginger, youre probably wrong

To determine the relationship between DG (Gibbs free energy change) and DGf° (standard Gibbs free energy change), we need to consider the formula for calculating DG for a reaction:

DG = ΣnDGf°(products) - ΣmDGf°(reactants)

In this reaction: MgF2(s) ===> Mg2+(aq) + 2 F-(aq)

We have one reactant, which is MgF2(s), and two products, Mg2+(aq) and 2 F-(aq). Therefore, the equation becomes:

DG = DGf°[Mg2+(aq)] + 2 DGf°[F-(aq)] - DGf°[MgF2(s)]

So, your first equation is correct:

DG = {DGf°[Mg2+(aq)] + 2 DGf°[F-(aq)] - DGf°[MgF2(s)]} + RT ln([Mg2+][F-]2)

Make sure to also remember that the R in the equation represents the gas constant and T represents the temperature in Kelvin. The ln([Mg2+][F-]2) term takes into account the effect of the concentration of the products.