A solution is made containing 20.8 g of phenol (C₆H₅OH) in 425 g of ethanol (CH₃CH₂OH). Calculate (c) the molality of phenol.

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Identify the formula for molality: \( m = \frac{\text{moles of solute}}{\text{kilograms of solvent}} \).

Calculate the moles of phenol (C_6H_5OH) using its molar mass. The molar mass of phenol is approximately 94.11 g/mol.

Convert the mass of phenol to moles by dividing the given mass (20.8 g) by the molar mass of phenol.

Convert the mass of ethanol (solvent) from grams to kilograms by dividing 425 g by 1000.

Substitute the moles of phenol and the kilograms of ethanol into the molality formula to find the molality of the solution.

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주요 개념

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Molality

Molality is a measure of the concentration of a solute in a solution, defined as the number of moles of solute per kilogram of solvent. It is expressed in units of moles per kilogram (mol/kg). Unlike molarity, which depends on the volume of the solution, molality is based solely on the mass of the solvent, making it particularly useful in situations where temperature changes may affect volume.

Molar Mass

Molar mass is the mass of one mole of a substance, typically expressed in grams per mole (g/mol). To calculate molality, it is essential to determine the molar mass of the solute—in this case, phenol (C6H5OH)—to convert the mass of the solute into moles. The molar mass is calculated by summing the atomic masses of all the atoms in the molecular formula.

Solvent and Solute

In a solution, the solute is the substance that is dissolved, while the solvent is the substance that does the dissolving. In this question, phenol is the solute and ethanol is the solvent. Understanding the roles of solute and solvent is crucial for calculating properties like molality, as it directly influences the amount of solute relative to the mass of the solvent.

A solution is made containing 20.8 g of phenol (C6H5OH) in 425 g of ethanol (CH3CH2OH). Calculate (c) the molality of phenol.