Question

Arrange the following aqueous solutions in the order of their increasing boiling points
(i) $10^{-4}MNaCl$
(ii) $10^{-3}M$ Urea
(iii) $10^{-3}MMgC{l}_2$
(iv) $10^{-3}MNaCl$

• 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) $10^{-4}MNaCl$
$\pi =iCRT$
$\left(NaCl⟶N{a}^{+}+C{l}^{-}\right)$
$\Rightarrow 2\times 10^{-4}RT$
(ii) $10^{-3}M$ Urea $\pi =1\times 10^{-3}RT$
(iii) $10^{-3}MMgC{l}_2\pi =3\times 10^{-3}RT$
$M{g}^{+2}+2C{l}^{-}$
$(i=3)$
(iv) $10^{-3}MNaCl$
$\pi =2\times 10^{-3}RT$
Now i increases $\Rightarrow \pi$ increases $=\Delta T_b$ increases $\Rightarrow \left(T_b-T_b^{\circ }\right)$ increases $=T_b$ increases
i.e. $T_b\propto i$
$(i)<(ii)<(iv)<(iii)$