Question

List the following solutions in order of decreasing freezing point: I. 0.040 mol kg−1 glycerin (a nonelectrolyte) II. 0.025 mol kg−1 Na2SO4 (assume complete dissociation in aqueous solution) III. 0.030 mol kg−1 KBr (assume complete dissociation in aqueous solution)

Answer 1

Answer: 0.040 mol glycerin > 0.030 mol KBr> 0.025 mol
`Na2SO_4`

Step-by-step explanation:

Depression in freezing point:

`T_f^0-T_f=i* k_b* m`

where,

`T_f` = freezing point of solution

`T^o_f` = freezing point of solvent

`k_b` = freezing point constant

m = molality

1. For 0.040 mol glycerin

, i= 1 as it is a non electrolyte and does not dissociate, concentration of ions will be
`1* 0.040=0.040`

2. For 0.025 mol
`Na2SO_4`

`Na_2SO_4\rightarrow 2Na^++SO_4^(2-)`

, i= 3 as it is a electrolyte and dissociate to give 3 ions. and concentration of ions will be
`3* 0.025=0.075`

3. For 0.030 mol KBr

, i= 2 as it is a electrolyte and dissociate to give 2 ions, concentration of ions will be
`2* 0.030=0.060`

Thus as concentration of solute is highest for
`Na_2SO_4` , the depression in freezing point will be highest and thus freezing point will be lowest.

The order of decreasing freezing point: 0.040 mol glycerin > 0.030 mol KBr> 0.025 mol
`Na2SO_4`