Question

Which has the lowest freezing point?
A).pure water
B).0.5 M ionic NaCl
C).0.5 M ionic CaCl2
D).0.5 M ionic AlCl3
E).0.5 M molecular C12H22O11

Answer 1

Answer: 0.5 M ionic $AlCl_3$

Explanation: Addition of nonvolatile solute depresses the freezing point of a solvent. As depression in freezing point is a colligative property, which means it depends on the amount of solute. Thus more is the no of ions, more is the depression in freezing point.

$\Delta T=k_f\times m$

$\Delta T$=Depression in freezing point

$k_f$= freezing point constant

m= molality

a) pure water: It is a pure solvent and freezes at 0 ° C.

b) 0.5M ionic NaCl:

$NaCl\rightarrow Na^++Cl^-$, it produces two ions, each of concentration 0.5M. The total concentration of ions is 1.0 M.

c) 0.5 M ionic $CaCl_2$

$CaCl_2\rightarrow Ca^{2+}+2Cl^-$, it produces two ions, $Ca^{2+}$ of concentration 0.5M and $Cl^-$ of concentration $2\times 0.5 M=1.0 M$. The total concentration of ions is 1.5 M.

d) 0.5 M ionic $AlCl_3$

$AlCl_3\rightarrow Al^{3+}+3Cl^-$, it produces two ions, $Al^{3+}$ of concentration 0.5 M and $Cl^-$ of concentration $3\times 0.5 M=1.5 M$The total concentration of ions is 2.0 M.

e) 0.5 M molecular $C_{12}H_{22}O_{11}$

It is a covalent compound and do not dissociate into ions. thus the concentration will be 0.5 M only.

As the concentration of ions is highest in 0.5 M ionic $AlCl_3$, the depression in freezing point is highest ans thus it has lowest freezing point.