Answer
$X_{C_4H_4S}=0.0393$
Work Step by Step
Mole fraction is moles solute / total moles.
First, calculate moles of thiophene (molar mass = 84.14 g/mol) and moles of toluene (molar mass = 92.14 g/mol) present.
$\frac{8.10\;g\;C_4H_4S}{}(\frac{1\;mol}{84.14\;g})=0.09627\;mol\;C_4H_4S$
$\frac{250.0\;mL\;C_7H_8}{}(\frac{0.867\;g}{1\;mL})(\frac{1\;mol}{92.14\;g})=2.352\;mol\;C_7H_8$
Next, calculate the mole fraction.
$X_{C_4H_4S}=\frac{moles\;C_4H_4S}{moles\;C_4H_4S+moles\;C_7H_8}=\frac{0.09627\;mol\;C_4H_4S}{0.09627\;mol\;C_4H_4S+2.352\;mol\;C_7H_8}=0.0393$
