How to Determine the Molecular Weight of a Compound from the Colligative Properties of a Solution – Part 1

Colligative properties are those properties of solutions which depend on the ratio of the number of solute particles to that of solvent particles in a solution, and not on the nature of the chemical species present.


The van 't Hoff factor i is a measure of the effect of a solute on colligative properties, viz., relative lowering in vapor pressure, boiling-point elevation, freezing-point depression and osmotic pressure, and is the ratio of the actual (experimental/observed) concentration of particles produced when the substance is dissolved to the formal (theoretical/normal) concentration that would be expected from its chemical formula. For most non-electrolytes dissolved in water, the factor is basically 1.

Question:

K4[Fe(CN)6] is supposed to be 40% dissociated when 1 M solution is prepared. Its boiling point is equal to that of another solution of 20% w/v of A (non-electrolyte). Considering Molality m = Molarity M, what the molecular weight of A?

Solution:



Click/tap to enlarge the image

α = Degree of dissociation, ΔTb = Boiling point elevation, Kb = Ebullioscopic constant

Curious about colligative properties❓ Let Chemaficionado know in the comments below or at mychemistryhomework@gmail.com❗

References:

(1)    Colligative properties. Wikipedia. https://en.wikipedia.org/wiki/Colligative_properties.

(2)    Wikipedia Contributors. van 't Hoff factor. Wikipedia. https://en.wikipedia.org/wiki/Van_%27t_Hoff_factor.

(3)    Ncert. Chemistry : Textbook for Class XII - Part.I; National Council Of Educational Research And Training: New Delhi.

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