[Molthisok]

.10 mol NaF
.10 mol IF_3
.10 mol HF

My best guess was IF_3 because it had the largest molar mass, but my professor’s key says I’m wrong

[AWK]

Why do you consider IF₃ to be a stable ionic compound in the water? You will never get an aqueous solution of this compound - it decomposes completely at -28°C.

[Borek]

The answer could depend on what are the the IF₃ decomposition products. However, there is definitely an error in your reasoning, as

because it had the largest molar mass

molar mass doesn't matter here at all.

[AWK]

The chemistry of interhalogen compounds is very far from general chemistry.
IF₃ at -28°C disproportionates to form iodine and iodine pentafluoride. IF₅ is fairly stable but reacts vigorously with water to form iodic acid (HIO₃) and hydrofluoric acid (this reaction can be found in Wikipedia).

[OrganicH2O]

If we ignore the IF3 decomposition, we could say:

NaF is an ionic compound with full water solubility. Each mole of NaF creates one mole of F- in water.

HF is a weak acid, so 1 mole of HF will dissociate partially to create less than one mole of HF is water.

IF3 is a covalent compound, so it will not dissociate in water at all, making zero moles of F- in water.

The molar mass does not matter because have already been given moles as your unit.

[AWK]

If we ignore the IF3 decomposition, we could say:

IF3 is a covalent compound, so it will not dissociate in water at all, making zero moles of F- in water.

This is chemical nonsense. We will never introduce IF₃ into the water, but possibly the equivalent (taking into account fluorine) amount of IF₅ that decompose in the water giving 0.3 M HF.

[OrganicH2O]

If we ignore the IF3 decomposition, we could say:

IF3 is a covalent compound, so it will not dissociate in water at all, making zero moles of F- in water.

This is chemical nonsense. We will never introduce IF₃ into the water, but possibly the equivalent (taking into account fluorine) amount of IF₅ that decompose in the water giving 0.3 M HF.

If the professor is asking a question like this, I assume this is the answer they want. For all we know, the person assigning the question doesn't even know IF3 is unstable. Generally, non-acidic and non-basic covalent compounds do not make ions in water. I assume based on the fact that this is undergraduate general chemistry forum.