Answer
See the answer below.
Work Step by Step
a) $d=PM/RT$
$M=5.60\ g/LĂ0.082Ă 291.2\ K/0.971\ atm$
$M=137.81\ g/mol$
Which is closest to $P_2F_4$'s molecular weight.
b) $0.050\ mol/min/0.028\ mol/min=\sqrt{M/44.0\ g/mol}$
$M=140.31\ g/mol$
Which is again closest to $P_2F_4$'s molecular weight.