If Pdcb replaces naphthalene as an active ingredient in mothballs , what is the value of kf for camphor if a 0.260 m solution of PDCB in camphor decreases the freezing point of camphor by 9.8 degrees C.

I am sure that you will use the formula ^T=-kfm. Since you are given the 0.260m, do you still have to calculate the molar mass for the solution of naphthalene?

I think the Kf for camphor is the Kf for camphor, regardless of what it's mixed with. If I understand the question, replacing naphthalene means we no longer need to worry about naphthalene in the solution. If I have misunderstood the question, please clarify.