If 25ml of bromine, a liquid element at room temp is added to 100ml of water, what would be the resulting molar concentration of bromine?

What's the density of liquid Br2?

25 mL x density = grams.
moles = grams/molar mass
M = moles/L of soln.
The technical answer is no one knows because we don't know that the volumes are additive; in fact, I suspect they are not additive. However, to make things simple I would add the 25 mL to 100 mL water to make 125 mL of soln. To do it exactly one would need the density of the solution.