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Ch.13 - Solutions & Their Properties
Chapter 13, Problem 103

What is the vapor pressure in mm Hg of a solution of 16.0 g of glucose (C6H12O6) in 80.0 g of methanol (CH3OH) at 27 °C? The vapor pressure of pure methanol at 27 °C is 140 mm Hg.

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1
Calculate the number of moles of glucose (C_6H_{12}O_6) using its molar mass. The molar mass of glucose is approximately 180.18 g/mol. Use the formula: \( \text{moles of glucose} = \frac{\text{mass of glucose}}{\text{molar mass of glucose}} \).
Calculate the number of moles of methanol (CH_3OH) using its molar mass. The molar mass of methanol is approximately 32.04 g/mol. Use the formula: \( \text{moles of methanol} = \frac{\text{mass of methanol}}{\text{molar mass of methanol}} \).
Determine the mole fraction of methanol in the solution. The mole fraction of methanol (\( X_{\text{methanol}} \)) is given by: \( X_{\text{methanol}} = \frac{\text{moles of methanol}}{\text{moles of methanol} + \text{moles of glucose}} \).
Apply Raoult's Law to find the vapor pressure of the solution. Raoult's Law states that the vapor pressure of the solution (\( P_{\text{solution}} \)) is equal to the mole fraction of the solvent (methanol) times the vapor pressure of the pure solvent: \( P_{\text{solution}} = X_{\text{methanol}} \times P^0_{\text{methanol}} \), where \( P^0_{\text{methanol}} \) is the vapor pressure of pure methanol.
Substitute the known values into the equation from Raoult's Law to calculate the vapor pressure of the solution.

Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Vapor Pressure

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid or solid phase at a given temperature. It reflects the tendency of particles to escape from the liquid phase into the vapor phase. The higher the vapor pressure, the more volatile the substance. In solutions, the presence of solutes can lower the vapor pressure compared to the pure solvent.
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Raoult's Law

Raoult's Law states that the vapor pressure of a solvent in a solution is directly proportional to the mole fraction of the solvent in the solution. This law helps predict how the addition of a solute affects the vapor pressure of the solvent. It is essential for calculating the vapor pressure of solutions, especially when dealing with non-volatile solutes like glucose in this case.
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Mole Fraction

Mole fraction is a way of expressing the concentration of a component in a mixture, defined as the number of moles of that component divided by the total number of moles of all components in the mixture. It is a crucial concept for applying Raoult's Law, as it allows for the calculation of the proportion of the solvent and solute in the solution, which directly influences the vapor pressure.
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