Answer
$12.8\%$
Work Step by Step
Number of moles of $KMnO_4$:
$0.0123\ M\times 22.25\ mL=0.274\ mmol$
From stoichiometry: $1.37\ mmol$ of iron
Mass:
$0.00137\ mol\times 55.845\ g/mol=0.0764\ g$
Mass fraction:
$0.0764/0.598\times 100\%=12.8\%$
