Consider 1.60 g of naphthalene (C10H8) is dissolved in 20.0 g of
benzene. The freezing point of benzene is 5.5oC, and the freezing point of the
mixture is 2.8oC. What is the molal freezing point constant for benzene?

Question

Consider 1.60 g of naphthalene (C10H8) is dissolved in 20.0 g of
benzene. The freezing point of benzene is 5.5oC, and the freezing point of the
mixture is 2.8oC. What is the molal freezing point constant for benzene?

Henry Antrim · Accepted Answer

Your experimental molal freezing point depression constant for benzene is #"4.3 °C·kg·mol"^"-1"#.

The formula for calculating freezing point depression is #color(blue)(|bar(ul(color(white)(a/a) ΔT_"f" = K_fm color(white)(a/a)|)))" "# where #ΔT_"f"# is the decrease in freezing point #K_"f"# is the molal freezing point depression constant #"m"# is the molality of the solution. We can rearrange the above formula to get #K_"f" = (ΔT_"f")/m# #ΔT_"f" = T_"f"^° -T_"f" = "5.5 °C – 2.8 °C" = "2.7 °C"# #"Moles of C"_10"H"_8 = 1.60 color(red)(cancel(color(black)("g C"_10"H"_8))) × ("1 mol C"_10"H"_8)/(128.17 color(red)(cancel(color(black)("g C"_10"H"_8)))) = "0.012 48 mol C"_10"H"_8# #"Molality" = "moles of solute"/"kilograms of solvent" = "0.012 48 mol"/"0.0200 kg" = "0.6242 mol/kg"# #K_"f" = (ΔT_"f")/m = "2.7 °C"/"0.6242 mol/kg" = "4.3 °C·kg·mol"^"-1"# The actual value of #K_"f"# for benzene is #"5.1 °C·kg·mol"^"-1"#. Are you sure that the observed freezing point wasn't 2.3 °C?

Kai Bowman · Answer

undefined The molal freezing point constant for benzene is 5.12 °C kg/mol.

Consider 1.60 g of naphthalene (C10H8) is dissolved in 20.0 g of benzene. The freezing point of benzene is 5.5oC, and the freezing point of the mixture is 2.8oC. What is the molal freezing point constant for benzene?

Consider 1.60 g of naphthalene (C10H8) is dissolved in 20.0 g of benzene. The freezing point of benzene is 5.5oC, and the freezing point of the mixture is 2.8oC. What is the molal freezing point constant for benzene?

Consider 1.60 g of naphthalene (C10H8) is dissolved in 20.0 g of
benzene. The freezing point of benzene is 5.5oC, and the freezing point of the
mixture is 2.8oC. What is the molal freezing point constant for benzene?