How many grams of S were used? What is the freezing point of S in degrees C?

A solvent, S, having a density of 1.06 g/mL and Kf = 3.90 ºC/m was uniformly cooled. A graph of temperature-time readings showed a plateau (flat) region [S(l)/S(s)] at 16.7 ºC. A solution of 2.49 g compound B was dissolved in 21.6 mL of S. I was cooled and dropped in temperature until reaching 3.38 degrees C.

1 Answer
Jose
Feb 18, 2018

#"Mass of Solvent"# = #"22.896 g"#

#"Freezing point = 16.7 ºC"#

Explanation:

#"Mass of Solvent"# = #"Volume x density"#

#"Mass of Solvent"# = #"21.6 mL" (1.06 g/(mL))# = 22.896 g
Or,

#"Mass of Solvent"# = #"0.023 kg"#

The Freezing point is read from the plateau zone which represents the Solid-Liquid Solvent equilibrium:

#"Freezing point = 16.7 ºC"#

IN ADDITION #=># BLAGDEN'S LAW
evaluates the Freezing Point Solvent Depression (#Delta T#) as function of its Solute Concentration (for an Ideal Solution):

#"Delta T# = #i K M#

where,

#i# : Van't Hoff Factor

#K# : Cryoscopic constant #(("ºC kg")/(mol))#
#M# : Molality of solute #("mol of solute"/("kg of solvent"))#

Assuming that is not an electrolyte; i.e. "i = 1"

#(16.7 - 3.38) ºC# = #(1) (3.9 ºC) ##(("mol of solute")/("0.022896 kg"))#

#"mol of solute"# = 0.0782

#"Molecular Weight of solute"# = 2.29 g / 0.0782 mol = 29.28 g/mol

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