How many moles of liquid water must freeze to remove 100 kJ of heat? (ΔHf = - 334 J/

g.

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