Question

Q4. (3 marks) Estimate the freezing point of 150 cm of water to which 13.0 g of sucrose (molar mass = 342.29 g/mol) has been added. Kf of water is 1.86 K kg mol-? Q5. (5 marks) The ideal solubility of solute B at temperature T is given by AfusH1 In Xg RT Where Arush is the enthalpy of fusion of solute and Te is the freezing point of solute. Estimate the ideal solubility of lead in bismuth (as kg of Pb in kg of Bi) at 250 °C. Melting point of Pb is 327°C and its enthalpy of fusion is 5.2 kJ/mol. Molar mass of Pb = 207.2 g/mol and molar mass of Bi is 208.98 g/mol Can the solution be described as Pb dissolved in Bi?

Answer 1

1) here we need to calculate freezing point change when sucose is added

ie freezing point elevation $\Delta$ Tf = Kf * m

Kf(freezing point elevation constant)=1.86 K.Kg/mol

m(molality)=moles of solute(sucose)/kg of solvent(water)

moles of solute=mass/molar mass = 13gm/(342.29gm/mol) =0.038mol

kg of solvent = 150cm3 =0.15Kg

then molality= 0.038mol/0.15kg =0.25mol/Kg

$\Delta$Tf = 1.86 K.Kg/mol * 0.25mol/kg =0.465K