Methane and oxygen react to form carbon dioxide and water. what mass of water is formed if 6.4 g of methane reacts with 25.6 g of oxygen to produce 17.6 g of carbon dioxide?

Number of moles of water = mass of water/ molar mass.  
 Molar mass of H2O = (1*2) + 16 = 18.0 
  Now we only need to calculate the number of moles of water.  
 The chemical reaction is as follows 
 CH4 + 2O2 ---> CO2 + 2H2O 
 From the eqn, the moles of H2O = 2*moles of CH4 
 Molar mass of CH4 = 12 + 4 = 16. Number of moles of methane = mass/
molar mass.   No of moles = 6.4/16 = 0.4.  So the number of mole of water = 2* 0.4 = 0.8. We already calculate its molar mass earlier.  So mass of water = molar mass * number of moles = 18 * 0.8 = 14.4