CH4 + 2O2 --> OC2 + 2H2O

What mass of water is formed by burning 4g of methane in excess oxygen?

(Ar of carbon=12, Ar of oxygen=16 and Ar of hydrogen=1)

1 answer

You transposed some letters; I assume you meant CO2 and not OC2.

mols CH4 = grams/molar mass = ?
Using the coefficients in the balanced equation, convert mols CH4 to mols H2O.
Then convert mols H2O to grams. grams H2O - mols H2O x molar mass H2O

Post your work if you get stuck.