Based on the tiles shown, how many moles of oxygen gas (o2) are needed to produce 36.04 grams of water (h2o) when reacting with excess ethane in the equation 2c2h6 7o2 → 5co2 6h2o?

We calculate for the number of moles of water given its mass by dividing the given mass by the molar mass. 
                               n water = (36.04 g) / (18 g/mol) 
                                n water = 2 mols
From the given balanced equation, every 6 moles of water produced will require 7 moles of oxygen.
                                n oxygen = (2 mols H2O) x (7 moles O2 / 6 moles H2O)
                                n oxygen  = 2.33 mols O2

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