Synthesis, Boiling Point

You dissolve 10.0 grams of a non-volatile solute in 200.0 g of benzene (Kb​=2.53∘C⋅kg/mol), and the boiling point of the solution rises by 1.5°C. What is the molecular mass of the solute?

m = Tb / kB → 1.5 / 2.53 =0.593 mol/kg

m = moles of solute / kg of solute

0.593 mol/kg × 0.2 kg = 0.1186 moles

Molar mass = 10.0 g / 0.1187 moles = 84.2 g/mol

A 1.80 g sample of an unknown non-electrolyte compound is dissolved in 75.0 g of benzene. The boiling point of the solution increases by 0.870°C. Given that the boiling point elevation constant (Kb​) for benzene is 2.53°C·kg/mol, determine the molecular mass of the unknown compound.

m = Tb / kb → 0.870 / 2.53 =0.344 mol/kg

m = moles of solute/kg of solute

0.344 mol * 0.0750 kg = 0.0258 moles

Molar mass = 1.80 g / 0.0258 moles = 69.8 g/mol

Why is an oxidizing agent required in this reaction?

• Cobalt(II) (Co²⁺) needs to be oxidized to cobalt(III) (Co³⁺) to form the stable [Co(NH₃)₆]³⁺ complex.

• This oxidation enhances the stability of the coordination complex, as Co³⁺ prefers strong ligand binding in an octahedral geometry.

How does the molecular structure of a synthesized compound affect its boiling point elevation in solution?

• Larger molecules tend to have higher intermolecular forces (e.g., hydrogen bonding, dipole interactions), influencing solubility and boiling point elevation.

• The presence of ionic solutes can lead to dissociation, requiring the Van’t Hoff factor (ii) in calculations (ΔTb=iKbmΔTb​=iKb​m), which accounts for multiple particles in solution.

How would the boiling point change if the solute were an electrolyte instead of a non-electrolyte?

• Electrolytes dissociate into multiple ions, increasing the number of solute particles in solution. This leads to a higher boiling point than a non-electrolyte of the same concentration. The Van’t Hoff factor (ii) accounts for this in calculations.

How does the boiling point elevation depend on the number of solute particles rather than their identity?

• Boiling point elevation is a colligative property, meaning it depends only on the quantity (concentration) of solute particles, not their chemical nature. More solute particles cause a greater effect.

Why does adding a non-volatile solute to a solvent increase its boiling point?

• When a non-volatile solute is added, it lowers the solvent’s vapor pressure. Since boiling occurs when the vapor pressure equals the external pressure, a higher temperature is needed to reach this point, increasing the boiling point.

How can boiling point elevation be used to determine the molecular mass of an unknown substance?

• By measuring how much the boiling point increases, we determine the molality of the solution. Since molality depends on the number of moles of solute, we can use the solute’s mass and solve for its molecular mass.

What is the purpose of the suction vacuum?

It quickly separates solids from liquids

• it leaves the solid behind while the liquid is able to go through the filter paper + funnel & into the beaker