Methane and oxygen react to form carbon dioxide and water. what mass of water is formed if 0.80 g of methane reacts with 3.2 g of oxygen to produce 2.2 g of carbon dioxide?

The Balance Chemical Equation is as follow,

                                CH₄  + 2 O₂   →   CO₂  +  2 H₂O

As 1 mole of CH₄ is equal to 16 g, then 0.8 g is equal to 0.05 moles.
Also, if 2 moles of O₂ is equal to 64 g, then 3.2 g is equal to 0.1 moles.
Now, moles of CO₂ and H₂O can also be calculated from equation. 
So, reacting 0.05 moles of CO₂ and 0.1 moles of O₂ will produce 0.05 moles (2.2 g) of CO₂ and 0.1 moles ( unknown mass) of H₂O)

Calculating for mass of H₂O,
As,
                             2 moles of H₂O is equal to   =   36 g of H₂O
Then
                   0.1 moles of H₂O will be equal to   =   X g of H₂O

Solving for X,
                                      X  =  (0.1 mol × 36 g) ÷ 2 mol

                                      X  =  1.8 g of H₂O is produced

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