Question

Would a solution of 1.0M NaCl or 1.0M MgCl₂ have a larger change in it's freezing point depression? Why?

Answer 1

The freezing point depression for 1M MgCl2 is larger than that of 1M NaCl.

Freezing point depression is defined as

ΔT_f =K_f​​​​​ × m × i

Here K_{​​​​​​f}​​​= vant Hoff factor, m = molality

i= No. of ions.

but to use that equation we need the molality of the solution which is moles of solute in kilograms of solvent.

Here as both MgCl2 and NaCl are 1M of equal solution so molality (m) is constant. Also vant Hoff factor (K_{​​​​​​f}) is also constant. Now freezing point depression depends only on i value.

If i value is more then freezing point depression is more.

For NaCl( Na⁺ + Cl^{​​​​​-}​) the value of i = 2

For MgCl2 ( Mg^{​​​​​2+} + Cl^{​​​​​-}​) the value of i = 3.

So freezing point depression is larger for MgCl2 than NaCl.