Answer
$-68.21\ kJ/mol$
Work Step by Step
From the definition of heat capacity:
$(125+50.0)\ mL\times 1.00\ g/mL\times 4.2\ J/g.K\times (24.40-21.50)\ K=2131.5\ J$
Exothermic reaction since the temperature increases.
Number of moles of CsOH:
$0.250\ M\times 125\ mL=31.25\ mmol$
Number of moles of HF:
$0.625\ M\times 50\ mL=31.25\ mmol$
Both reactants are in stoichiometric proportions.
Enthalpy per mole:
$-2131.5\ J\div 31.25\ mmol=-68.21\ kJ/mol$
