Question

What is meant by chelate effect ? Give an example.

Hint:

Chelating (ring-forming) ligands add stability vs monodentate ones.

Answer 1

Concept: When you dissolve a non-volatile solute (like ethylene glycol) in a solvent, the freezing point of the solvent drops. How much it drops is given by $\Delta T_f = K_f \times m$, where $m$ is the molality (moles of solute in one kg of solvent) and $K_f$ is a constant for the solvent.

Step 1: Find the moles of ethylene glycol
Moles is just mass divided by molar mass: \[ n = \frac{31}{62} = 0.5\ \text{mol} \]

Step 2: Find the molality
Molality is moles of solute per kg of solvent. The water is $600\ \text{g}$, which is $0.600\ \text{kg}$. So: \[ m = \frac{0.5\ \text{mol}}{0.600\ \text{kg}} = 0.833\ \text{mol kg}^{-1} \]

Step 3: Find the drop in freezing point
Now put the numbers into the formula: \[ \Delta T_f = K_f \times m = 1.86 \times 0.833 = 1.55\ \text{K} \] This $1.55$ is how much the freezing point falls.

Step 4: Find the new freezing point
Pure water freezes at $0\,°\mathrm{C}$. We subtract the drop: \[ \text{Freezing point} = 0 - 1.55 = -1.55\,°\mathrm{C} \] which is the same as about $271.6\ \text{K}$.

Answer: The freezing point falls by $\Delta T_f = 1.55\ \text{K}$, so the solution freezes at $-1.55\,°\mathrm{C}$.