This is a boiling point elevation problem in which you use the boiling point elevation to determine the molar mass of the solute.
Step 1. Determine the molar mass.
We use the boiling point elevation expression
#ΔT_"b" = K_"b"m#
where# ΔT_"b"# is the change in boiling point, #K_"b"# is the molal boiling point elevation constant for the solvent, and #m# is the molality of the solute.
For benzene, #K_"b"# = 2.53 °C·kg·mol⁻¹ and #T_"b"^#° = 80.10 °C.
#ΔT_"b" = T_"b" – T_"b"^°# = 80.78 °C – 80.10 °C = 0.68 °C