The reaction ag+(aq) + cl−(aq) ↔ agcl(s) has an equilibrium of 1020 (keq = 1020). if you have a beaker containing 1 liter of water with 0.1 mol of agcl(s) in it along with 10−6 m ag+(aq) and 10−15 m cl−(aq), how does the reaction shift?

When Q is equal the initial concentration of the products / the initial concentration of the reactants. 

so, Q = [Ag]*[Cl-]  and we neglected [AgCl] as it is solid

∴ Q = 10^-6 * 10^-5 

       = 10^-11 

now we will compare the value of Q with the value of Keq:

when Q = Keq so, the system is in equilibrium

and when Q > Keq so, the reaction will go forward (shift to right) to achieve equilibrium.

and when Q< Keq so, the reaction will go left (shift to left) to achieve equilibrium.

when Q = 10^-11 and Keq = 10^20

∴Q< Keq 

and the reaction will shift to left.

Potassium and fluorine combine to form potassium fluoride having the formula kf