your confusing me, first you use n in the equation and now your using m.

It depends on how C is given.

C

_{M} = q / n?T ----> Molar heat cap

C = q /m?T ----> Specific heat cap

But I thought you would already be aware of that ...

In the problem, "Assume that the specific heat of H2O(l) is constant at 4.184 J/K.g"

This means C of water = q / m?T . So q =mC?T

?S

_{sys} = mCpLn(Tf / Ti) or ?S

_{sys} = nCpLn(Tf / Ti)

You should use one of them depending on the units of Cp. Since Cp is given in J/K.grams, then we use the 2nd form.

I know how to find the dS for the cold water warming up and the hot water cooling down, I just don't know how to find the dS for the ice melting.

You already know how much heat is abrosbed by ice, and at temp 273.15 K, so u can easily determine the change in entropy.