Answer: 86,9g
Explanation: Molar mass of C is 12, of H it's 1. Molar mass of butane is 4*12 + 10*1 (C4H10) = 58g/mol. So 56g of butane is (56/58) =0,9655mol.
2C4H10 + 13O2 = 8 CO2 + 10 H2O.
From 2 mol butane you can make 10 mol H2O.
So if you only have 0,9655 mol butane, then that will give you a maximume of (0,9655/2)*10=4,828 mol water. 1 mol water's mass is 18g. So 4,828 mol is 4,828*18=86,89g.