[dontfailplz]

I have come across a question and the answer required the knowledge that SrF is insoluble, while AgF is and I was wondering why.

[Enthalpy]

Does SrF exist?

It's only a part of the answer. The rest is complicated.

[Corribus]

As I understand, SrF₂ is the common fluoride of strontium - strontium, being a group 2 element, typically forms a +2 cation.

@OP - Solubility rules are complicated. In principle, the solubility can be understood in terms of the thermodynamics (enthalpy and entropy) of (1) stabilizing the ions in the aqueous phase and (2) overcoming the lattice energy that holds together the solid. (1) often is related to the charges and relative sizes of the formed ions. (2) often is related to similar factors, although for somewhat different reasons.

By inspection alone, solubility trends can be difficult to explain. Some insight can occasionally be gained by looking up and comparing thermodynamic values for all the species involved in the dissolution equilibria - e.g., heats of formation and standard entropies of the ions, lattice energies/entropies, etc.. Although this only often yields information on what factors influence the relative solubilities, not why they do. E.g., you may find (just for sake of argument - I didn't look for real values) that strontium fluoride has a higher lattice energy than silver fluoride, which could explain why it is less likely to dissolve in water. Pinpointing the reason why this would be the case isn't necessarily easy, though. With so many factors involved, there usually isn't 1 single reason why one compound is more or less soluble than another, and trends down a group or across a period can be very complicated as a resolut.

To give an idea of the complexity of understanding solubility rules, check out the following page on group 2 carbonates:

https://www.chemguide.co.uk/inorganic/group2/problems.html