Answer
See the answer below.
Work Step by Step
Number of moles of $I_2$:
$0.246\ M\times 40.21\ mL/1000 \ mL/L=0.00989\ mol$
From stoichiometry:
$0.00989\ mol\times 2/1=0.01978\ mol$ thiosulfate.
Mass of sodium thiosulfate:
$0.01978\ mmol\times 158.10\ g/mol=3.127\ g$
Mass percentage:
$3.127/3.232\times 100\%=96.76\%$
