Textbook Question
Indicate whether each statement is true or false: (d) If you take a saturated solution and raise its temperature, you can (usually) add more solute and make the solution even more concentrated.
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Ch.13 - Properties of Solutions
Chapter 13, Problem 42c
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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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
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.
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Molality
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.
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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.
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Suppose that one wishes to use reverse osmosis to reduce the salt content of brackish water containing 0.22 M total salt concentration to a value of 0.01 M, thus render- ing it usable for human consumption. What is the mini- mum pressure that needs to be applied in the permeators (Figure 18.20) to achieve this goal, assuming that the oper- ation occurs at 298 K? (Hint: Refer to Section 13.5.)
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