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Chapter 13: Problem 46

Explain why molality is used for boiling-point elevation and freezing-point depression calculations and molarity is used in osmotic pressure calculations.

Short Answer

Expert verified
Molality is used for boiling-point elevation and freezing-point depression calculations because these properties are independent of temperature and pressures changes, they purely depend on the amount of solute in kg of solvent temperature. Molarity is used in osmotic pressure calculations because osmotic pressure is 'volume dependent', it requires the amount of solute per unit volume of solution.

Step by step solution

01

Understand Molality

Molality is a measure of the concentration of a solute in a solution in terms of amount of substance in a specified amount of mass of the solvent. This makes molality a useful property since it is independent of temperature and pressure unlike other units of concentration.
02

Understand Molarity

Molarity is a measure of the concentration of a solute in a solution, or of any chemical species, in terms of amount of substance in a specified volume of solution. The volume dependence on temperature and pressure for molarity is a key factor.
03

Why Molality for Boiling and Freezing Point Calculations?

Boiling-point elevation and freezing-point depression depend on the molality of the solute particles since these properties depend on the number of solute particles in the solvent and not on the amount of solution. Molality remains constant with temperature because it involves the mass of the solvent (in kg), which doesn't change with temperature.
04

Why Molarity for Osmotic Pressure Calculations?

Osmotic pressure depends on the molarity of the solution because it is 'pressure' caused by the solute particles per unit volume of solution. Since molarity varies with temperature and pressure as volume changes, it accurately captures the effect on osmotic pressure.

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Most popular questions from this chapter

Explain each of these statements: (a) The boiling point of seawater is higher than that of pure water. (b) Carbon dioxide escapes from the solution when the cap is removed from a soft-drink bottle. (c) Molal concentrations and molar concentrations of dilute aqueous solutions are approximately equal. (d) In discussing the colligative properties of a solution (other than osmotic pressure), it is preferable to express the concentration in units of molality rather than in molarity. (e) Methanol (b.p. \(65^{\circ} \mathrm{C}\) ) is useful as an auto antifreeze, but it should be removed from the car radiator during the summer season.

A man bought a goldfish in a pet shop. Upon returning home, he put the goldfish in a bowl of recently boiled water that had been cooled quickly. A few minutes later the fish was dead. Explain what happened to the fish.

Which of these two aqueous solutions has (a) the higher boiling point, (b) the higher freezing point, and (c) the lower vapor pressure: \(0.35 \mathrm{~m} \mathrm{CaCl}_{2}\) or \(0.90 \mathrm{~m}\) urea? State your reasons.

Define ion pairs. What effect does ion-pair formation have on the colligative properties of a solution? How does the ease of ion-pair formation depend on (a) charges on the ions, (b) size of the ions, (c) nature of the solvent (polar versus nonpolar), (d) concentration?

Outline the steps required for conversion among molarity, molality, and percent by mass.
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