An unknown substance is known to be a nonelectrolyte,and it dissolves in water.Determine the approximate molar mass of the substance if 37.7 g of the substance dissolved in 1.00 × 10² mL of water has a boiling temperature of 102.73°C.K_b for water is 0.52°C/m. A)5.3 g/mol B)1.4 g/mol C)2.0 × 10² g/mol D)3.8 × 10² g/mol E)72 g/mol

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Quizplus · Accepted Answer

The boiling point elevation formula is $\Delta T_b = iK_b m$, where $\Delta T_b$ is the change in boiling point, $i$ is the van't Hoff factor (which is 1 for nonelectrolytes), $K_b$ is the ebullioscopic constant, and $m$ is the molality of the solution. Given $\Delta T_b = 102.73°C - 100°C = 2.73°C$, $K_b = 0.52°C/m$, and the mass of the substance is 37.7 g dissolved in 1 kg of water, we can solve for the molar mass. First, find the molality: $2.73°C = (1)(0.52°C/m)(m)$, so $m = 2.73°C / 0.52°C/m = 5.25 m$. Since molality is moles of solute per kilogram of solvent, and we have 1 kg of solvent, the moles of solute is 5.25. The molar mass is then $37.7 g / 5.25 mol = 7.2 g/mol$, which rounds to approximately 72 g/mol.

An Unknown Substance Is Known to Be a Nonelectrolyte,and It