Answer
See the answer below.
Work Step by Step
a) $d=P\bar{M}/RT$
$\bar{M}=0.092\ kg/m^3×8.314\ Pa.m^3/mol.K×210\ K\div (42/760×101325\ Pa)$
$\bar{M}=0.0287\ kg/mol=28.7\ g/mol$
b) $28.7\ g/mol=28.01×x+32.00×(1-x)$
$x=0.827=82.7\%$
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