Question

Which of these solutions will have highest boiling point?

1) 0.20 m glucose
2) 0.10 m Na2SO4
3) 0.25 m sucrose
4) 0.10 m NaCl

Answer 1

Answer: 2) 0.10 m $Na_2SO_{4}$

Explanation:

Elevation in boiling point is a colligative property which depends on the amount of solute.

$\Delta T_b=i\times k_b\times m$

$\Delta T_b$ =  elevation in boiling point

i = Van'T Hoff factor  

$k_f$ = boiling point constant

m = molality

1. For 0.20 m glucose

i= 1 as it is a non electrolyte and does not dissociate.

Thus concentration of ions = 0.20

2. For 0.10 m $Na_2SO_{4}$

i= 3 as it is a electrolyte and dissociate to give 3 ions.

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

Thus concentration of ions = $3\times 0.10=0.30$

3. For 0.25 m sucrose

i= 1 as it is a non electrolyte and does not dissociate.

Thus concentration of ions = 0.25

4. For $NaCl$

i= 2 as it is a electrolyte and dissociate to give 2 ions.

$NaCl\rightarrow Na^{+}+Cl^{-}$  

Thus concentration of ions = $2\times 0.10=0.20$

Thus as  concentration of ions is highest for $Na_2SO_4$, the boiling point will be highest.