Propane, C3H8, is a fuel used in such devices as camp stoves. If the molar heat of combustion of propane is 2200 kJ/mol, what mass, in grams, of propane would be needed to heat 2.0 L of water from 25.0°C to 100.0°C?

First, calculate the heat needed to heat the water.

q = mass H2O x specific heat H2O x (Tfinal-Tinitial)
Plug in the numbers from the problem and calculate q. Convert to kJ
Next, translate how much heat is needed to how much propane is required to produce that much heat.
C3H8 + 5O2 --> 3CO2 + 4H2O delta H = 2200 kJ/mol C3H8
So you know that 44 g C3H8 will produce 2200 kJ
44 x (q kJ from above/2200 kJ) = grams propane needed.
Post your work if you get stuck.