Step 1: Convert the given amount to moles, by dividing the given amount by the molecular weight of the compound.
5 g C3H8 (1 mol / 44 g C3H8) = 0.114 mol C3H8
Step 2: Find the mole ratio between the given and the desired target (in this case water). The mole ratio between C3H8 and water is 1:4, that is, you can make 4 times as much water from a one C3H8 (assuming that it isn't the limiting reactant).
0.114 mol C3H8 (4 moles H2O / 1 mol C3H8) = 0.456 mol H2O
Step 3: Reconvert back to grams by multiplying by the molecular weight of the new target (water).
0.456 mol H2O (18 g / 1 mol H2O) = 8.208 g Water. Hope this helped! :)
