if 15 grams of ethane gas c2h6, reacts with 128 g oxygen un a combustion reaction, how many moles of water can be produced?

Answer 1

This is a limiting reagent (LR) problem and you know that because amounts are given for BOTH reactants.
2C2H6 + 7O2 ==> 4CO2 + 6H2O

mols C2H6 = grams/molar mass = ?
mols O2 = grams/molar mass = ?

Using the coefficients in the balanced equation, convert mols ethane to mols H2O
Do the same and convert mols O2 to mols H2O.

It is likely that the two values for mols H2O will not be the same which means one is not correct; the correct value in LR problems is ALWAYS the smaller value and the reagent responsible for that value is the LR.