To determine the number of moles of Be, we need to divide the given mass by its molar mass.
The atomic mass of Be is 9.012 g/mol.
Number of moles of Be = mass of Be ÷ molar mass of Be
Number of moles of Be = 50.4 g ÷ 9.012 g/mol
Number of moles of Be = 5.592 mol
Therefore, you have 5.592 moles of Be.