Question

If 5.0 moles of C3H8 react, how many molecules of water are formed?

Answer 1

 Propane is C3H8 

The balanced equation is: 
C3H8 + 5 O2 --> 3 CO2 + 4 H2O 
From the balanced equation you can see that for each mole of C3H8 you obtain 4 moles of H2O. 

moles H2O = 5.0 moles C3H8(4 moles H2O/1 mole C3H8) = 20 moles H2O formed 

Use Avogadro's number to find the number of molecules. 
molecules H2O = 20 moles H2O(6.02 x 10^23 molecules/1 mole) = 1.20 x 10^25 H2O molecules