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