13.44 g of CuCl2 is dissolved in 1 kg of water. Assuming complete dissociation of CuCl2, the elevation in the boiling point of water is (Kb of water = 0.52 K kg mol−1, molar mass of CuCl2 = 134.4 g mol−1)
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 0.16 K
Elevation of boiling point is a colligative property. For an electrolyte the van’t Hoff factor i counts the particles each formula unit gives:
\( \displaystyle \Delta T_b=i\,K_b\,m \)
Moles of solute: \( \displaystyle n=\frac{13.44\,\mathrm{g}}{134.4\,\mathrm{g\,mol^{-1}}}=0.1\,\mathrm{mol} \)
Molality: \( \displaystyle m=\frac{0.1\,\mathrm{mol}}{1\,\mathrm{kg}}=0.1\,\mathrm{mol\,kg^{-1}} \)
van’t Hoff factor: \( \ce{CuCl2 -> Cu^2+ + 2Cl^-} \), so complete dissociation gives i = 3.
\( \displaystyle \Delta T_b=3\times0.52\,\mathrm{K\,kg\,mol^{-1}}\times0.1\,\mathrm{mol\,kg^{-1}} \)
\( \displaystyle \Delta T_b=0.156\,\mathrm{K}\approx0.16\,\mathrm{K} \)