Chapter Nine Solutions Fundamentals of General, Organic, and Biological Chemistry 7th Edition Chapter 9 Lecture © 2013 Pearson Education, Inc. McMurry, Ballantine, Hoeger, Peterson

Copyright © Cengage Learning. All rights reserved 2 Solution – homogeneous mixtures of molecules and ions – Solvent – substance present in largest amount – Solute(s) – other substance(s) in the solution – Aqueous solution (aq) – solution with water as the solvent Colloids are homogenous mixtures with particles ranging in diameter nm – They scatter light 9.1 Mixtures and Solutions Recall that homogenous implies uniform mixing with the same composition throughout.

A colloid example is milk. It has tiny globules of fat dispersed in the liquid. They are apparent under a microscope.

Tro, Chemistry: A Molecular Approach The Solution Process A solution forms when the solvent-solute interaction forces are stronger than (or at least comparable to) the solute-solute interaction forces. If the solute-solute interactions are much stronger, the solute will not dissolve.

“Like Dissolves Like” Nonpolar compounds dissolve in nonpolar solvents – e.g. hexane or oil solvent Polar (and ionic) compounds dissolve in polar solvents – e.g. salt and sugar in water – Solute hydrogen bonding also contributes to water solubility Review polarity from Ch 4! – Small asymmetric molecules are polar, and thus water soluble Recall each water molecule makes 4 hydrogen bonds to other water molecules. The oil is “squeezed out” of the aqueous phase.

Pure hydrocarbons (C & H only) The above large pure hydrocarbons follow their smaller cousins like methane, in being nonpolar – Review London dispersion forces and IMFs from Ch 8! Large polar molecules typically have hydrogen bonding sites…

Ethanol is miscible (soluble in any proportion) with water due to the dominance of it’s own polar OH bond – An infinitely soluble substance is called miscible – Ethanol’s miscibility is caused by the availability of two hydrogen bonds per ethanol molecule 7 Glucose (C 6 H 12 O 6 ) above is somewhat soluble in water due to 6 hydrogen bonding sites (-OH). The hydrocarbon “backbone” is, however, too large to be [completely] miscible with water.

Solubility of Ionic Substances Ionic substances like NaCl break up into individual cations and anions – Cation (Na + ) bonds to  - pole on water – Anion (Cl - ) bonds to  + pole on water Solvation or hydration occurs when water molecules form a large electrostatic shell around each ion Copyright © Cengage Learning. All rights reserved 8

Miscible: Mutually soluble in all proportions – Ethyl and methyl alcohols will continue to dissolve in water no matter how much are added But most substances reach a solubility limit beyond which no more will dissolve Solubility: The maximum amount of a substance that will dissolve in a given amount of solvent at a specified temperature Only 9.6 g baking soda dissolves in 100 mL water at 20  C Compared to 206 g sucrose under same conditions 9.4 Solubility

A saturated solution is in a state of dynamic equilibrium. Dissolution and crystallization occur at the same nonzero rates. Saturated solution: Contains the maximum amount of dissolved solute at equilibrium – No more than 35.8 g of NaCl will dissolve in 100 mL of water at 20  C – Any amount above this limit sinks to the bottom of the container as a solid

Temperature often has a dramatic effect on solubility, but is very unpredictable In Intro, most solids become more soluble as temperature rises, while the solubility of gases always decreases – but check out NaCl and Ce 2 (SO 4 ) Effect of Temperature on Solubility

Supersaturated solutions have solute levels above the equilibrium solubility level. Prepared by heating to increase solubility and dissolution… followed by cooling to lower the equilibrium solubility level. – Excess solute remains dissolved (supersaturation) – And precipitates dramatically when disturbed Extra info: supersaturated solutions

c = kP at constant T – Liquid conc (c) and gas pressure (P) It can also be explained using Le Châtelier’s principle – When the system is stressed by increasing the pressure of the gas, more gas molecules go into solution to relieve the increased pressure – When the pressure of the gas is decreased, more gas molecules come out of solution to relieve the decrease 9.6 Pressure and Solubility: Henry’s Law

Worked example 9.3 At a partial pressure of oxygen in the atmosphere of 159 mmHg, the solubility of oxygen in blood is 0.44 g/100 mL. What is the solubility of oxygen in blood at 11,000 ft, where the partial pressure of O 2 is 56 mmHg? According to Henry’s law, the solubility of the gas divided by its pressure is constant:

1) For solid solutions (eg, brass) concentrations are typically expressed as mass/mass percent concentration, (m/m)%: 2) For liquid solutions, concentrations are expressed as volume/volume percent concentration, (v/v)%: 9.7 Units of Concentration

3) A third ‘percent’ method is to give the number of grams of solute as a percentage of the number of milliliters – This is mass/volume percent concentration, (m/v)%: 4) Molarity (M) is used in chemistry. It represent the moles of solute dissolved into total solution volume, which includes both the solvent and the solute.

Many solutions are stored in high concentrations and then prepared for later use by dilution The amount of solute remains constant; only the volume is changed by adding more solvent Because the number of moles remains constant, we can set up the following dilution equation for molarity: 9.8 Dilution

© 2013 Pearson Education, Inc. Concentrated ammonia solution is available in a 16.0 M solution. What volume must be used to prepare mL of a 2.0 M solution? a.94 mL b.43 mL c.9.4 mL d.4.3 mL

Strong electrolytes are substances that ionize completely when dissolved in water – NaCl  Na + (aq) + Cl - (aq) – HCl  H + (aq) + Cl - (aq) Weak electrolytes are substances that are only partly ionized in water – Eq shifted left toward CH 3 COOH (acetic acid) Nonelectrolytes are substances that do not produce ions when dissolved in water – C 6 H 12 O 6 (Glucose) – Water is a nonelectrolyte Ion levels are small (1 x M) for pure water (sec 10.6) 9.9 Ions in Solution: Electrolytes

Electrical conductivity of aqueous solutions Many ions – large current Few ions – weak current No ions – no current

gram-equivalent (g-Eq) is a practical unit – The amount of ions (in grams) that actually contains that one mole of charge Ion(s) that have a +1 or −1 charge – so 1 gram-equivalent of Na + is simply 23.0 g – so 1 gram-equivalent of Cl − is simply 35.4 g Ion(s) that have a +2 or −2 charge – so 1 gram-equivalent of Mg 2+ is (24.2/2) = 12.2 g – so 1 gram-equivalent of S 2− is (32.1/2) = 16.0 g 9.10 Electrolytes in Body Fluids

Note the narrow ranges for these electrolytes critical for supporting human health. Significant figures/rounding can have serious implications!!!

Worked Example 9.14 The normal concentration of Ca 2+ in blood is 5.0 mEq/L – Clinical chemists use milliequivalents (1000 mEq = 1 Eq) How many milligrams of Ca 2+ are in 1.00 L of blood?

9.12 Osmosis and Osmotic Pressure Osmosis: The passage of a solvent through a semipermeable membrane separating two solutions of different concentration.

– M = molar concentration (mol/L) – R = universal gas const – T = Kelvin temperature The osmotic pressure of a 0.15 M NaCl solution at 25 °C is 7.3 atm – It has units of pressure Osmotic pressure depends only on the concentration of solute particles Osmolarity (osmol) is the sum of the molarities of all dissolved particles in a solution

Osmosis is particularly important in living organisms because the membranes around cells are semipermeable. Water, but not larger molecules, passes through the membrane. – Isotonic: Having the same osmolarity – Hypotonic: Having an osmolarity less than the surrounding blood plasma or cells. – Hypertonic: Having an osmolarity greater than the surrounding blood plasma or cells. Hypertonic medium- water exits cell Isotonic medium- cell fluid volume const Hypotonic medium- water enter cells and they rupture

Low osmolality = good hydration beverage

© 2013 Pearson Education, Inc. When dissolved in enough water to make 1.0 liter of solution, which of the following solutes yields the solution with the highest osmolarity? a.0.10 mole of CaCl 2 b.0.15 mole of NaCl c.0.30 mole of glucose d.All three of the above solutions have the same osmolarity.