Answer
$ -3.090 \ MJ/mol$
Work Step by Step
From the definition of heat capacity:
$(775\ g\times 4.184\ J/g.K+893\ J/g)\times (31.69-22.50)\ K=38.0\ kJ$
Since it's an exothermic process, the sign is negative.
Number of moles of benzoic acid:
$1.50\ g\div 110.11\ g/mol=0.0136\ mol$
Hence:
$\Delta U=-38.0\ kJ/0.0123\ mol=-3.090 \ MJ/mol$
