Question

Which of the following solutes will lower the freezing point of water the most?

the molecular compound sucrose (C12H22O11)
the ionic compound magnesium sulfate (MgSO4)
the ionic compound lithium chloride (LiCl)
the ionic compound calcium fluoride (CaF2)

Answer 1

Freezing point depression is directly proportional to molality. So the compound to have the greatest effect will be the one that disassociates into the most ions = CaF2

Answer 2

Answer: the ionic compound calcium fluoride
`(CaF_2)`

Step-by-step explanation:

`\Delta T_f=i* k_f* m`

`T_f` = change in freezing point

i = Van'T Hoff factor

`k_f` = freezing point constant

m = molality

1. For
`C_(12)H_(22)O_(11)` , i= 1 as it is a non electrolyte and does not dissociate.

2. For
`MgSO_(4)` , i= 2 as it is a electrolyte and dissociate to give 2 ions.

`MgSO_4\rightarrow Mg^(2+)+SO_4^(2-)`

3. For
`LiCl`, i= 2 as it is a electrolyte and dissociate to give 2 ions.

`LiCl\rightarrow Li^(+)+Cl^(-)`

4. For
`CaF_(2)`, i= 3 as it is a electrolyte and dissociate to give 3 ions.

`CaF_2\rightarrow Ca^(2+)+2F^(-)`

Thus as vant hoff factor is highest factor for
`CaF_(2)` and the freezing point will be lowest.