Question

List the following solutions in order of decreasing freezing point: I. 0.040 mol kg−1 glycerin (a nonelectrolyte) II. 0.025 mol kg−1 Na2SO4 (assume complete dissociation in aqueous solution) III. 0.030 mol kg−1 KBr (assume complete dissociation in aqueous solution)

Answer 1

Answer:  0.040 mol glycerin > 0.030 mol KBr>  0.025 mol $Na2SO_4$

Explanation:

Depression in freezing point:

$T_f^0-T_f=i\times k_b\times m$

where,

$T_f$ = freezing point of solution

$T^o_f$ = freezing point of solvent

$k_b$ = freezing point constant

m = molality

1. For 0.040 mol glycerin

, i= 1 as it is a non electrolyte and does not dissociate, concentration of ions will be $1\times 0.040=0.040$

2. For 0.025 mol $Na2SO_4$

$Na_2SO_4\rightarrow 2Na^++SO_4^{2-}$  

, i= 3 as it is a electrolyte and dissociate to give 3 ions. and concentration of ions will be $3\times 0.025=0.075$ 

3. For 0.030 mol KBr

, i= 2 as it is a electrolyte and dissociate to give 2 ions, concentration of ions will be $2\times 0.030=0.060$

Thus as concentration of solute is highest for $Na_2SO_4$ , the depression in freezing point will be highest and thus freezing point will be lowest.

The order of decreasing freezing point: 0.040 mol glycerin > 0.030 mol KBr>  0.025 mol $Na2SO_4$