Apparatus

A. Temperature-controlled magnetic stirrer
B. Thermometer
C. Beaker (500 mL)
D. Test tube
E. Naphthalene
F. Sulfur powder
G. Clamp

Procedure in laboratory (diagram)










Depression of freezing point,
∆T_f=-(T_f¹-T_f)

Procedure in laboratory (diagram)



See the video!

Data and the analysis
Determination of freezing point of pure naphthalene and naphthalene-solute mixture





Calculation


Figure 1. Temperature vs time graph of pure naphthalene and naphthalene-solute mixture.

The cryoscopic constant (K_f) of naphthalene is 6.9 °C/molality. From the experiment, we measured the depression in the freezing point (ΔT_f) of naphthalene after adding the unknown solute is 5 °C. For the experiment 5 gm (w_solvent)of naphthalene was taken and 1 gm (w_solute)of unknown solute was added in naphthalene. According to equation 5, the molecular weight of the solute added in the naphthalene solvent will be M=K_f×w_solute×1000/ΔT_f×wsolvent

M=(6.9×1×1000)/(5×5)=276 g mol^(-1)

Procedure in simulator of the experiment

To be added based on the simulation

Analysis

From the experiment, the molecular weight of the unknown solute is measured as 272 g/mol. In this experiment, rhombic sulphur (S8) was used as the unknown solute which has an actual mass of 256 g/mol.