From the chemical equation we see that 1 mole of diborane (B2H6) will produce 2 moles of water.
We will firstly calculate the moles of diborane:
[tex]\begin{gathered} _nB_2H_6=\frac{mass}{molar\text{ }mass} \\ \\ _nB_2H_6=\frac{3.7g}{27.7\text{ }gmol^{-1}} \\ _nB_2H_6=0.134\text{ }moles \end{gathered}[/tex]One mole of diborane produces 2 moles of water then to find the moles of water that is produced we:
[tex]\begin{gathered} _nH_2O=2\times0.134\text{ }mole \\ _nH_2O=0.268 \end{gathered}[/tex]We then convert the moles of the water to mass:
[tex]\begin{gathered} mass=moles\times molar\text{ }mass\text{ }H_2O_ \\ \\ mass=0.268\text{ }mol\times18\text{ }gmol^{-1} \\ \\ mass=4.824g \end{gathered}[/tex]Mass of water is 4.824g