Answer
$-27.3\ kJ/mol$
Work Step by Step
Number of moles of lead nitrate:
$200.\ mL\times 0.75\ M=150\ mmol$
Number of moles of NaBr:
$200.\ mL\times 1.5\ M=150\ mmol$
Both are in stoichiometric proportions.
From the definition of heat capacity:
$(200+200)\ mL\times 1.0\ g/mL \times 4.2\ J/g.K \times 2.44\ K=4099.2\ J$
With a negative sign since it's exothermic.
Per mol:
$-4099.2\ J/150\ mmol=-27.3\ kJ/mol$
