# Chapter 5 Principles of Chemical Reactivity: Energy and Chemical Reactions - Study Questions - Page 217g: 99

#### 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: $6.3\ kJ\div 352.9\ kJ/mol\times24.305\ g/mol=0.43\ g$

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