Answer
See the answer below.
Work Step by Step
The number of moles of Nitrogen is:
$n=(713/760×1\ atm)×0.295\ L/((0.082)(294\ K))$
$n=0.0115\ mol$
From stoichiometry:
$0.0115\ mol\ NaNO_2$
Which corresponds to:
$0.0115\ mol×69.0\ g/mol=0.791\ g$
Mass fraction:
$0.791/1.232×100\%=64.2\%$
