Question

Assuming equal concentrations and complete dissociation, rank these aqueous solutions by their freezing points. nh4cl na3po4 li2so4

Answer 1

By using the formula of depression in freezing, it can easily identify the rank of the given compounds.

$\Delta T= i.K_{f}.m$

where,

$\Delta T$ = freezing point depression

i = van 't Hoff factor (number of ions per individual molecule of solute)

m = molality of the solute

$K_{f}$ = molal freezing point

According to question, if concentration (m) and $K_{f}$ is same, then only van 't Hoff factor will change.

For $NH_{4}Cl$, i = 2

For $Na_{3}PO_{4}$, i = 4

For $Li_{2}SO_{4}$, i= 3

Now, the solute with the largest i value results in low freezing point.

The highest value of i is 4 ($Na_{3}PO_{4}$), thus  $Na_{3}PO_{4}$ has lowest freezing point.

And, $NH_{4}Cl$ has highest freezing point.

The order of freezing point is : $NH_{4}Cl> Li_{2}SO_{4}> Na_{3}PO_{4}$