Answer
See answer below.
Work Step by Step
Enthalpies of formation (kJ/mol):
$H_2O(l):-285.83, Mg(OH)_2(s):-924.54$
Heat of reaction:
$-924.54-2\times-285.83=-352.9\ kJ/mol$
Energy required to heat the water:
$25\ mL\times 1.00g/mL\times 4.184\ J/g.K \times(85-25)\ K=6276\ J$
Mass of magnesium:
$6.3\ kJ\div 352.9\ kJ/mol\times24.305\ g/mol=0.43\ g$
