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?

1 Answer
Ernest Z.
May 11, 2016

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

Explanation:

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?

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