Question

Arrange the following aqueous solutions in the order of their increasing boiling points
(i) $10^{-4} \,M\, NaCl$
(ii) $10^{-3} M$ Urea
(iii) $10^{-3} \,M\, MgCl _{2}$
(iv) $10^{-3} \,M\, NaCl$

• A. (i) < (ii) < (iv) < (iii)
• B. (ii) < (i) = (iii) < (iv)
• C. (i) < (ii) < (iii) < (iv)
• D. (iv) < (iii) < (i) = (ii)

Answer 1

Answer: (A)

(i) $10^{-4} \,M \,NaCl $
$\pi= iCRT$
$\left( NaCl \longrightarrow Na ^{+}+ Cl ^{-}\right) $
$\Rightarrow 2 \times 10^{-4} RT$
(ii) $10^{-3} M$ Urea $\pi=1 \times 10^{-3} RT$
(iii) $10^{-3} M MgCl _{2} \pi=3 \times 10^{-3} RT$
$Mg ^{+2}+2 Cl ^{-}$
$( i =3)$
(iv) $10^{-3} \,M \,NaCl$
$\pi=2 \times 10^{-3} \,RT$
Now i increases $\Rightarrow \pi$ increases $=\Delta T_{b}$ increases
$\Rightarrow \left(T_{b}-T_{b}^{\circ}\right)$ increases $=T_{b}$ increases
i.e. ${T_{b} \propto i}$
$(i) < (ii) < (iv) < (iii)$