Question

True or False: The freezing point of a 0.15 MgCl2 (aq) should theoretically be lower than the freezing point of a 0.25m KI (Aq). Explain why

Answer 1

No. It is not true.

The lowering of a freezing point is expressed as:

$\delta T=iC_mK$

,where i - vant Hoff factor, which is equal to the number of ions produced by one formula unit of the salt (if the dissociation is complete). Thus, i(MgCl₂) = 3 and i(KI) = 2:

$MgCl_2 \rightarrow Mg^{+2}+2Cl^-$

$KI \rightarrow K^++I^-$

Finally,

$\delta T(MgCl_2)=3*0.15*K=0.45K$

$\delta T(KI)=2*0.25*K=0.50K$