Question

Data were collected to determine the molar mass of a nonvolatile solid solute dissolved in cyclohexane. Complete the table for the analysis. Record calculated values with the correct number of significant figures. Show detailed calculations to the right of the table. Volume of cyclohexane (mL): 15.5 Mass of cyclohexane (kg): _________ Mass of added solute (g): 0.1660 Kf for cyclohexane (˚C/m): 20.0 Freezing point change, ΔTf (˚C): 2.20 Moles of solute (mol): _________ Molar mass of solute (g/mol): _________

Answer 1

From the depression in freezing point

∆Tf=i x Kf X m

Where ∆Tf=depression in freezing point, i=Van't Hoff factor=1 for non volatile solid, Kf=freezing point constant and m= molality=moles of solute/mass of solvent.

Given ∆Tf=2.20°C, Kf=20°C/m, mass of solute=0.166 g

Volume of cyclohexane=15.5 mL and density=0.779 g/mL

Mass of solvent=density X volume=15.5 ml X 0.779 g/ml=12..07 g

Mass of solvent=0.01207 Kg. (1kg=1000g)

Therefore ∆Tf=kf X (moles of solute/mass of solvent)

Moles of solute=(2.20 °C X 0.01207 Kg)/(20°C X Kg/mol)=1.328 X 10^-3 mol

Moles of solute=mass of solute/molar mass of solute

1.328 X 10^-3 mol =0.166 g/molar mass

Molar mass of solute=0.166 g/1.328x10^-3 mol=124.98 g/ mol.

Mass of cyclohexane=0.01207 Kg,

Moles of solute=1.328x10^-3 mol,

And molar mass of solute=124.98 g/mol.