Question

Which of these solutions has the lowest freezing point?

0.25 M NaCl
0.5 M NaCl
1.0 M NaCl
1.5 M NaCl
2.0 M NaCl

Answer 1

Answer : The correct option is, 2.0 M NaCl

Explanation :

Formula used for lowering in freezing point :

`\Delta T_f=i* k_f* m`

where,

`\DeltaT_f` = change in freezing point or freezing point depression

`k_f` = freezing point constant

m = molality

i = Van't Hoff factor

As we know that the Van't Hoff factor for NaCl will be same for all given concentrations of NaCl and
`k_f` is the constant term. So, freezing point depression directly depends only on the molality of the solution.

That means the more the value of molality, the lower will be the freezing point and vice-versa.

From the given options, 2.0 M NaCl has the lowest freezing point.

Hence, the correct option is, 2.0 M NaCl

Answer 2

Answer is: 2.0 M NaCl.

Change in freezing point from pure solvent (water) to solution (sodium chloride solution): ΔT = i · Kf · c.

Kf - molal freezing-point depression constant for water is 1.86°C/m.

c- molarity of the solution.

i - Van't Hoff factor.

Because molal freezing-point depression constant and Van't Hoff factor are the same for all five solutions, freezing point depends on molarity of the solution.

The higher is the molarity, the lower is freezing point.

The temperature at which the phase transition solid-liquid occurs is the melting point or the freezing point.