Answer
$17.7\%$
Work Step by Step
For 1 mol of mixture:
$0.20\ mol\ hexane \times 86.178\ g/mol=17.24\ g$
$0.80\ mol\ hexane \times 86.178\ g/mol=80.16\ g$
Mass fraction of hexane:
$\dfrac{17.24}{17.24+80.16}=0.177\ or\ 17.7\%$
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