Question

Which of the following aqueous solutions would have the highest boiling point? 3.0 M solution of molecular compound sucrose (C12H22O11) 3.0 M solution of ionic compound aluminum chloride (AlCl3) 3.0 M solution of ionic compound lithium bromide (LiBr) 3.0 M solution of ionic compound calcium fluoride (CaF2)

Answer 1

Answer: option b) 3.0 M solution of ionic compound aluminum chloride (AlCl₃)

Explanation:

1) The solutions given are:

a) 3.0 M solution of molecular compound sucrose (C₁₂H₂₂O₁₁)

b) 3.0 M solution of ionic compound aluminum chloride (AlCl₃)

c) 3.0 M solution of ionic compound lithium bromide (LiBr)

d) 3.0 M solution of ionic compound calcium fluoride (CaF₂)

2) The elevation of the boiling point of a solution is a colligative property.

3) That means that the elevation of the boiling point depends on the number of solute particles in the solvent and not the identity per se of the solute particles.

4) The formula of the elevation of the boiling point, ΔTb is:

ΔTb = i × m × Kb.

Where:

i) i is the Van't Hoof constant and is equal to the number of particles for each molecule or unit formula.

If the compound is a molecule (covalent bond) i = 1, if the compound is ionic, i is the number of ions product of the dissociation (I will show you how to determine it for each of the compounds given).

ii) m is the molality of the solution (moles of solute per kg of solvent)

iii) Kb is the molality boiling constant. It is a constant for every solvent. It does not change with the solute.

4) For our four compounds, m and kb are egual, so you must analyze the i factor for each solution.

a) 3.0 M solution of molecular compound sucrose (C₁₂H₂₂O₁₁)

Since it is a molecular compound it does not dissociate and i = 1

b) 3.0 M solution of ionic compound aluminum chloride (AlCl₃)

Assume the ionic compound dissociates 100%. The for eveary unit of AlCl₃ there will be 4 ions. Those are 4 particles and means that i = 4.

c) 3.0 M solution of ionic compound lithium bromide (LiBr)

Assuming also 100% dissociation of this ionic compound, since every LiBr unit dissociates into two ions, i = 2.

d) 3.0 M solution of ionic compound calcium fluoride (CaF₂)

Following same reasoning, i = 3.

5) Conclusion: since the greatest i factor is 4, and given all the other factors equal, the 3.0 M solution of ionic compound aluminum chloride (AlCl₃), option b, would be the aqueous solutions with the highest boiling point,

Answer 2

The value of boiling point depend on the chemical bond that holds the particles of the substance together and the molecular weight of that substance. Looking at the substances given in the question, the ionic compounds will have the higher boiling point and AICI3 which have the highest molecular mass will have the highest boiling point. Therefore the correct option is B.