Question

19. Which of the following aqueous solutions has the highest freezing point (assume 100% dissociation for all soluble ionic compounds)?

A) 0.10m Mg(NO3 )2 (an electrolyte)

B) 0.11m Na2SO3 (an electrolyte)

C) 0.17m MgCO3 (an electrolyte)

D) 0.18m NaCl (an electrolyte)

E) 0.29m C6H12O6 (not an electrolyte)

Answer 1

ΔTf = i*Kf*m

ΔTf is the depression in freezing point

ΔTf will be maximum when i*m is maximum

so,

freezing point will be lowest when i*m is maximum

freezing temperature is maximum when ΔTf will be minimum

i is the number of ions that a compound can break into

A)

Mg(NO3)2

i = 3

Mg(NO3)2 breaks into 1 Mg2+ ion and 2 NO3- ions

so,

i*m = 3*0.10 = 0.30

B)

Na2SO3

i = 3

so,

i*m = 3*0.11 = 0.33

C)

MgCO3

similarly:

i = 2

i*m =2*0.17 = 0.34

D)

NaCl

i = 2

i*m = 2*0.18 = 0.36

E)

C6H12O6

This is non electrolyte

i = 1

i*m = 0.29

i*m is lowest for E.

So, E will have lowest freezing point

E