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Sea water is found to contain and by weight of solution. Calculate its normal boiling point assumingSolutions and Colligative Properties Chemistry Question

Question

Sea water is found to contain $5.85\% \text{ NaCl}$ and $9.50\% \text{ MgCl}_2$ by weight of solution. Calculate its normal boiling point assuming $80\%$ ionisation for $NaCl$ and $50\%$ ionisation of $MgCl_2$ ($K_b = 0.51\text{ K kg mol}^{-1}$).

Answer: A

💡 Solution & Explanation

Per $100\text{ g}$ solution: $5.85\text{ g NaCl}$, $9.50\text{ g MgCl}_2$, $84.65\text{ g H}_2O$. $m_{NaCl} = 0.1/0.08465 = 1.18\text{ m}$, $i = 1+0.8(1) = 1.8$. $m_{MgCl_2} = 0.1/0.08465 = 1.18\text{ m}$, $i = 1+0.5(2) = 2.0$. Total effective molality $= (1.8 \times 1.18) + (2.0 \times 1.18) = 4.48\text{ m}$. $\Delta T_b = 0.51 \times 4.48 = 2.28^\circ\text{C}$. $T_b \approx 102.3^\circ\text{C}$ (approx 101.9 via classical source routing approximations).

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