Pure Benzene, C₆H₆, Has a Molar Mass of 78

Short Answer

Pure benzene, C₆H₆, has a molar mass of 78.114 g mol⁻¹, a density of 0.8765 g mL⁻¹, a freezing point of 5.45°C, and a boiling point of 80.2°C. Its freezing point depression and boiling point elevation constants are: K_f = 5.07°C m⁻¹; K_b = 2.53°C m⁻¹. A solution was made by taking 33.88 g of an unknown nonelectrolyte and dissolving it in 175.0 g of benzene. The measured freezing point of the solution was -1.65°C. Calculate the molar mass of the unknown substance.Hint: Find molality, then moles, then molar mass, being sure to keep track of units.