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Copyright © Houghton Mifflin Company. All rights reserved. 7–1

Solutions

Homogeneous Mixtures of Compounds


7.1 Characteristics of SolutionsDefinitions:

Solution: A homogeneous combination of two or more substances, in which each substance retains its chemical identity

Solvent: The component of a solution present in the greatest amount

Solute(s): The component(s) of a solution present in smaller amount(s) than the solvent


7.1 Characteristics of SolutionsSolutions have variable compositions; the

ratio of solvent to solute(s) can be varied.

The properties of the solution change as the ratio of solvent to solute(s) is varied.

Solutes are present as individual molecules, atoms, or ions.

The composition of a solution is constant through space.

Solute(s) can be separated from the solvent by physical means such as evaporation.


7.2 Solubility

Solids and liquids are usually more solu-ble at high temperatures than at low temperatures

Gases are usually more soluble at low temperatures than at high temperatures

Gases are more soluble at high pressures than at low pressures.


Table 7.1 Solubilities of Various Compounds in Water at 0C, 50C, and 100C.


7.2 Solubility

Saturated solutions contain the maximum amount of solute that the solvent can hold at a given temperature. Undis-solved solute is usually present.

Unsaturated solutions contain less than the maximum amount of solute that can be dissolved at a given temperature.


Figure 7.3 In a saturated solution, the dissolved sol-ute is in a dynamic equilibrium with the undissolved solute.


7.2 Solubility

Aqueous solutions have water as the solvent.

Nonaqueous solutions have substances other than water as the solvent.


7.3 Solution Formation

To form a solution, two sets of intermolecular forces must be overcome

Forces between solvent molecules

Forces between solute molecules or ions

Attractive forces between solute and solvent particles must compensate for disrupting the other forces.


Figure 7.5 Hydration of ions in an aqueous solution


7.4 Solubility Rules

Like dissolves like

In water, ionic compounds are more likely to be soluble if the charge on individual ions is low

NaCl (Na1+ + Cl1–) is soluble

Na3PO4 (3 Na1+ + PO43–) is soluble

Ca3(PO4 )2 (3 Ca2+ + 2 PO43–) is not soluble

Ca10(PO4)6(OH)2 is not soluble


Table 7.2 Solubility Guidelines for Ionic Compounds in Water.


7.5 Solution Concentration Units

Concentration: Amount of solute in a specified amount of

solution

Percent concentrationsPercent by mass (m/m)

Percent by volume (v/v)

Mass-volume percent (m/v)

Molarity: Moles Solute per Liter Solution



Molarity: Moles solute = mol = M Liter solution L

Molarity is a conversion factor, like molar mass, that allows us to convert between moles and an easily measurable quantity, volume.



What is the molarity of a solution of NaCl if 50.0 g of NaCl is dissolved in 1.00 L of water?

How many moles of NaCl are present in 250 mL of 0.100 M solution?

How many milliliters of 0.250 M NaCl will contain 0.500 mol of NaCl?



H3PO4 + 3 NaOH Na3PO4 + 3 H2O

How many milliliters of 0.0500 M NaOH will react with 15.0 mL of 0.0300 M H3PO4?

What is the molarity of H3PO4 if 15.05 mL of 0.1500 M NaOH react with 10.00 mL of the H3PO4 solution?



Percent by Mass: %(m/m), %(w/w)

Mass of solute x 100% Mass of solution

What is the percent by mass of a solution made by combining 8.5 g of sucrose with 73.8 g of water?

How many grams of sucrose and how many grams of water must be used to prepare 250 g of solution that is 5.0%(m/m)?


7.5 Solution Concentration UnitsPercent by Volume: %(v/v)

Volume of solute x 100% Volume of solution

What is the percent by volume of a solution in which 15 mL of glacial acetic acid is diluted to 100 mL with water?

Could you do this by combining 15 mL of acetic acid and 85 mL of water?


Figure 7.7 When volumes of two different liquids are combined, the volumes are not additive.


Figure 7.8 Volumetric flasks are filled to the 50.0 mL mark with ethanol and with water. The liquids are combined in a 100 mL volu-metric flask, and the volume is 96.5 mL.



Mass-volume percent: %(m/v)

Grams of solute x 100% Milliliters of solution

mg/dL Milligrams of soluteDeciliters of solution

1 mg = 0.001 g 1 dL = 100 mL


7.6 Dilution CalculationsIt is common to make dilute solutions from

concentrated solutions (a.k.a. stock solutions)

C1V1 = C2V2

A nurse must prepare 100 mL of 1.0 % (m/v) AgNO3. She has a 3.0% (m/v) stock solution. How much stock solution should she use? How should she prepare the solution?


7.7 Colligative Properties of Solutions

Colligative properties of a solution depend on the concentration of solute particles, not their chemical identity

Lowering of vapor pressure

Freezing point depression

Boiling point elevation

Osmotic Pressure


Figure 7.10 Close-ups of the surface of a solvent (a) pure liquid (b) with nonvolatile solute


7.7 Colligative Properties of Solutions

Nonvolatile solutes:

Lower the vapor pressure of the solution relative to that of the pure solvent

Raise the boiling point of the solution relative to that of the pure solvent

Lower the freezing point of the solution relative to that of the pure solvent.


7.8 Osmosis and Osmotic Pressure

Osmosis is the passage of solvent through a semipermeable membrane from a dilute solution or pure liquid to a concentrated solution.

Semipermeable membranes have very small pores, and allow only certain small molec-ules to pass from one side to the other.


Figure 7.12 (a) Osmosis, the flow of solvent through a semipermeable membrane from a dilute to a concentrated solution (b) At equilibrium, the molecules move back and forth at equal rates.


Figure 7.13 A semipermeable membrane separating(a) pure water and solute-water solution (b) dilute solute-water solution and a concentrated solute-water solution.


Figure 7.14Osmotic pressure is the amount of pressure needed to prevent the solution in the tube from rising as a result of the process of osmosis.


Isotonic, Hypertonic, and Hypotonic Solutions

Isotonic solutions have the same osmotic pressure as intracellular fluids

Red blood cells are stable

Hypertonic solutions have higher osmotic pressure that intracellular fluids

Red blood cells undergo crenation

Hypotonic solutions have lower osmotic pressure than intracellular fluids

Red blood cells undergo hemolysis


Isotonic, Hypertonic, and Hypotonic Solutions


7.9 Dialysis

Dialysis is the process in which a semi-permeable membrane permits the passage of solvent and small solute particles (ions and molecules) but blocks the passage of large molecules.

Cell membranes permit dialysis


Figure 7.17 In dialysis, there is a net movement of ions from a region of higher concentra-tion to a region of lower concentration. (a) Before dialysis. (b) After dialysis.


Figure 7.18 Impurities (ions) can be removed from a solution by using a dialysis procedure.

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