Answer
See the answer below.
Work Step by Step
a) From the data given:
$-25.4\ kJ\div 0.850\ g\times24.305\ g/mol=-726.3\ kJ/mol$
b) From the definition of heat capacity:
$25400\ J= (750\ g\times4.184\ J/g.K+820\ J/K)\times(T-18.6)\ K$
$25400=3958(T-18.6)$
$T=25.0^{\circ}C$
