To go from mL to grams, you'll also want the density of water, but that's so trivial that it's barely worth noting. My assumption is that they want you to go with the bog-standard 1 g/cm³ (at 4°C) rather than tracking its variability over different temperatures, which means that 150mL of water masses a convenient 150g, since it's a only a difference of something like 1.8E-3 grams for the difference between 4°C and 20°C. That's just me being pedantic, though, since you specifically mentioned the possibility of needing more physical constraints, and since you already corrected the error that put your poor 20°C ice cube under over a GPa of pressure.
Any rate, if I recall properly (and it's been even longer for me than TheBigger Fish), what you want to do is basically use conservation of energy - the energy that goes into melting and heating your ice to your equilibrium temperature
T is the same as the energy that cools the rest of the system from 20°C to the same equilibrium temperature
T. Regarding the ice, the first bit requires your heat of fusion; the second bit is a more conventional temperature change using the specific heat (of water, since it's no longer solid). Since the ice conveniently starts at its melting point of 0°C, you don't actually seem to need the specific heat of ice as far as I can tell; there is no thermal change that takes place while the ice is actually still solid, since it immediately melts. For
Q as the heat in the system and
q as the specific heats...
Qmelt +
Qwarming water +
Qcooling water +
Qcooling cup = 0
Qmelt +
Qwarming water = - (
Qcooling water +
Qcooling cup )
(
qfusion *
mice) + (
qwater *
mice * Δ
Tice) = - ( (
qwater *
mwater * Δ
Tcup and water) + (
qcup *
mcup * Δ
Tcup and water) )
For Δ
Tcup and water =
T - 20 and Δ
Tice =
T - 0 =
TShove stuff around and solve for
T - that's your equilibrium temperature where everything ends up at. At least, assuming I remember this right. I am a bit out of practice.
((Also, can I just take this opportunity to reiterate that customary units are horrifying? I dusted off my old thermal textbook for a quick refresher and was confronted by visions of
Btu/hr.))