First figure out how many grams must freeze and then convert the grams to moles.
Hf = -334 J/g. Convert this to KJ/g by dividing by 1000. (There are 1000 Joules in a kJ).
Hf = -334 J/g ÷ 1000 J/kj = -0.334 kJ/g
Now, divide 100 kJ by -0.334 kJ/g (see how the units are lining up?)
100 kJ ÷ -0.334 kJ/g = 299 g
Now convert this to moles by dividing by the molecular weight of water (18.0g/mole).
299 ÷ 18.0 = 16.6 moles
