Question

Sodium chloride is a 2-ion electrolyte, dissociating 90% in a certain concentration. Calculate the freezing point of a molal solution. [Density of 6% w/v sodiumchloride solution is 1.0457. Freezing point depression of 1% NaCl = -0.58°C ]

Answer 1

(a)On the basis of 90% dissociation, 100 particles of sodium chloride will yield:
90 zinc ions
90 sulfate ions
10 undissociated particles or
190 particles
Because 190 particles represent 1.9 times as many particles as were present before dissociation, the dissociation (i) factor is 1.9,answer.

Or use formula

$i=1+(n-1)@$

Here n= no of moles and @= dissociation degree

Given n=2

@=90%=90/100=0.9

So, $i=1+(2-1)(0.9)=1+0.9=1.9$

(b)For this we use a simple formula given as

ΔT=i*k_f*m

Where $k{_f}$ is a constant given as 1.86 for freezing point depression.

i is dissociation factor( calculated above)

ΔT=1.9*1.86*1=3.534

So, freezing point is -3.534°C