Chapter 12:Solutions

Chem 1110

Figures: Basic Chemistry 3rd Ed., Timberlake and Timberlake

Examples of Common Solutions

Solid Solutions:

• Brass (65% Cu, 35% Zn)• White Gold (60% Au, 40% Pt)

Liquid Solutions:

• Vinegar (95% water, 5% acetic acid) • Solutions used in lab

Gas Solutions:• Air (78% N2, 20% O2, 1% Ar, 0.5% CO2, 0.5% H2O)

Solutions

We need to define the important components in a solution:

• Solvent: The most abundant substance in a solution

• Solute: Any substance dissolved in the solvent

• Aqueous Solution: a solution of any solute in water. Water is the solvent.

Solid Solutions

Solid Solutions: An Alloy is a solid solution of metals:

• If one of the metals is mercury (Hg) we call this solution an amalgam

• Amalgams were commonly used in older dental work

• Brass (65% Cu, 35% Zn)• White Gold (60% Au, 40% Pt)

Solutions

Concentration is the amount of solute in a given volume of solution

• Concentrated Solution has a large amount of solute per volume

• Dilute Solution has a small amount of solute

• This is a Qualitative distinction, of course we will have to consider Quantitative values

Saturated Solutions

We can add solute to a solvent until NO more solid will dissolve to form a Saturated Solution

• Saturation defines the solubility of the solute

• Solubility is the maximum amount of solute that can be dissolved in a given solvent

Solubility is based on:

1. Pressure applied to the system

2. Temperature of the solution

3. Nature of the solvent and solute interaction

Solubility

Solubility increases as temperature increases:

Solubility in Water

Supersaturated Solutions

If we heat a saturated solution to dissolve more solute, we can create a Supersaturated Solution:

• Supersaturated Solutions are:

• An Unstable State because there is too much solute for the amount of solvent

• Will precipitate out very easily

• Used to make hot packs for injuries

Temperature and Pressure affect solubility:

• For solid or liquid solutes:

• Increasing T, increases solubility

• Supersaturated Solution

• Increasing P has NO effect on solubility

• For a gaseous solute:

• Increasing T, decreases solubility

• Increasing P, increases solubility

Henry’s Law

Solubility

Attractive Forces in Solutions

Interactions between solutes and solvents that:

• are similar in polarity will form a solution

• are not similar will not form a solution

“Like dissolves like”

Water as a Polar Solute

Solubility

Two liquids that dissolve into each other are said to be miscible

• If you consider Intermolecular Forces:

“Like dissolves like”

Ethanol and water are miscible

Hexane and water are immiscible

Like Dissolves Like

Solvents Solutes

Water (polar) Ni(NO3)2

CH2Cl2(nonpolar) (polar)

I2

(nonpolar)

Units of Concentration

We report the concentration of solutions in many different units:

• There are several important Career specific units to consider:

• Percent by Mass (m/m)

• Percent by Volume (v/v)

• Concentrations in chemistry are most often reported in Molarity (M)

Percent by Mass

Percent by Mass (m/m) is used most often in engineering or agricultural applications

• Commonly measured in grams

Percent by Mass = mass of solute x 100%

mass of solution

• 6.0 % (m/m): 6.0 g of solute dissolved in a total of 100 g of solution (6 g solute and 94 g solvent)

Percent by Mass

Low fat milk has 1% (m/m) milkfat

• This means that in this sample of milk there is…

© Brooks/Cole, Cengage Learning. All rights reserved.

Percent by Volume

Percent by Volume (v/v) is used most often when both solute and solvent are liquids or gases:

Percent by volume = volume of solute x 100%

volume of solution

• 17.0 % (v/v): 17.0 mL of solute dissolved in a total of 100 mL of solution (17 mL solute and 83 mL solvent)

Percent by Volume

Rubbing alcohol is 70% (v/v) isopropanol in a solution with water

• This means that in the bottle there are…

© Brooks/Cole, Cengage Learning. All rights reserved.

Molarity

Molarity (M) is the unit of concentration most often used in chemistry labs:

Molarity (M) = moles of solute

1 liter of solution

• A solution that has 1 mole of solute dissolved in a total of 1 liter of solution is said to be 1 molar (1 M)

Molarity (M) = moles

liter

Learning CheckHow many grams of CuSO4 (159.10 amu) are needed to make exactly 500 mL of a 1.00 M solution?

Learning Check

How many moles of CuSO4 are in 150 mL of the 1.00 M solution?

Dilution

We decrease the concentration (or dilute) aqueous solutions by adding water

• The moles of solute STAYS THE SAME

• The volume of the solution changes

• Results in a change of molarity (M)

Molarity decreases

DilutionIn a dilution:• water is added• volume increases• concentration decreases

Dilution

Initially:

Molarity (M1) x volume (V1) = # moles solute

After Dilution:

New molarity (M2) x volume (V2) = # moles solute

# moles solute = # moles solute

M1V1 = M2V2

V2 = V1 + Vadded

DilutionWhat is the molarity of a the final solution after diluting 150 mL of a 1.0 M CuSO4 solution with 200 mL of water?

Molarity in a Chemical Equation

How many milliliters of a 3.00 M HCl solution are needed to react with 4.85 g of CaCO3?

2HCl(aq) + CaCO3(s) CaCl2(aq) + CO2(g) + H2O(l)

Learning Check

How many liters of H2 gas at STP are produced when 6.25 g of Zn react with 20.0 mL of a 1.50 M HCl solution?

Zn(s) + 2 HCl(aq) ZnCl2 (aq) + H2(g)

Colligative Properties

Homogeneous solutions may have different physical properties than the pure substances that they contain

• Properties of substances in a solution may be different:

– We will talk about adding nonvolatile solute leads to

• Increased melting point• Increased boiling point• Decreased vapor pressure

Colligative Properties

Colligative Properties are properties of solutions that depend on the number of particles dissolved in the solvent

• Colligative Properties do not depend on the identity of the particle:

o IonsoCovalent Molecules

• It is the concentration of particles that matters, particle molarity

Colligative Properties

Dissolved Particles in a solution:

• 1 sugar molecule → 1 sugar molecule

o Total of 1 particle in solution

• 1 NaCl formula unit → 1 Na+ ion + 1 Cl- ion

o Total of 2 particles in solution

• (NH4)3PO4 →

Particle Molarity

Dissolved Particles in a solution:

• 1 M sugar molecule → 1 M sugar molecule

o Total of 1 M particles in solution

• 1 M NaCl formula unit → 1 M Na+ + 1 M Cl-

o Total of 2 M particles in solution

• 1 M (NH4)3PO4 → 3 M NH4+ + 1 M PO4

3-

o Total of 4 M particles in solution

Learning CheckWhat is the molarity of particles in a solution of 125 g of MgCl2, a strong electrolyte, dissolved in 0.500 L of water?

Colligative Properties

A large number of solute particles may cause interruptions or “get in the way” of the solution surface:

• Dissolved particles may disrupt intermolecular forces

• The pure substance is “diluted” by the solute

• Decrease solvent-solvent interactions on the surface

Colligative Properties

Solute particles (ionic or molecular) can:

• Interrupt the intermolecular forces holding the molecules of a solvent together

• Weaken the intermolecular forces of the solvent

• “Get in the way” on the surface of a solution and make it more difficult for molecules to escape

Colligative Properties

Colligative Properties may include:

1. Boiling Point Elevation

2. Freezing Point Depression

3. Osmotic Pressure

Colligative Properties

Effect on Boiling Point…

It is common to add salt to water when boiling, why?

• Adding a non-volatile solute may lower vapor pressure

• Ions interfere with solvent-solvent interactions on the surface

• Ions make it harder for water molecules to escape

– Fewer solvent (water) particles on the surface

Colligative Properties: Boiling Point Effects

• Ions make it harder for water molecules to escape

– Fewer solvent (water) particles on the surface

Colligative PropertiesBoiling Point Effects:

• A higher temperature is needed to get the vapor pressure to equal atmospheric pressure

• DECREASE the vapor pressure of the solution • INCREASE the boiling point of the solution• INCREASE number of particles for a larger effect

• Adding salt to water raise boiling temperature:

1 mole of particles raises boiling point by 0.52 °C

• Antifreeze in car engines raises boiling point in radiator

Colligative Properties

Effect on Freezing Point…

Why do we add salt to roads in winter?

• Salt interferes with intermolecular forces holding the H2O molecules together

• Water molecules are not able to organize into ice crystals since solute is in the way

• A lower temperature is needed to reach the solid state

• The Freezing Point is DECREASED

Colligative PropertiesWhy do we add salt to roads in winter?

• Salt interferes with intermolecular forces holding the H2O molecules together

• Water FREEZES at a lower temperature

• Salt prevents roads from re-freezing but it DOES NOT melt the ice!!

• CaCl2 (Ice Melter®) melts ice and lowers the melting temperature even colder

The oceans never freeze solid due to higher salt concentrations

Osmosis

OSMOSIS: the transport of a solvent across a semi-permeable membrane

• We will limit our discussion to water as the solvent

• Water moves across the membrane to create an isotonic system

Example of Osmosis

A semi-permeable membrane separates a 4% starch solution from a 10% starch solution. Starch is a colloid and cannot pass through the membrane, but water can. What happens?

10% starch4% starchH2O

Semi-permeable membrane

Example of Osmosis (continued)

The 10% starch solution is diluted by the flow of water out of the 4% solution, and its volume increases

• The 4% starch solution loses water, and its volume decreases

• Eventually, the water flow between the two becomes equal

7% starch H2O

7% starch

H2O

Osmotic Pressure

• Water flows from less concentrated to more concentrated side of a semi-permeable membrane

• Movement of water molecules sets up a pressure differential

• Osmotic Pressure is the pressure required to make these two sides equal equilibrate

Hypotonic, Hypertonic,Isotonic Solutions

• Maintaining appropriate solution conditions are very important in your body and chemistry

• Consider the possible negative effects…

Isotonic Fluid

Consider, an isotonic solution such as blood:

Blood plasma (liquid) should have the same osmotic pressure as in your red blood cells:

o 5.0% (m/v) glucose o 0.9% (m/v) NaCl

Red Blood Cell: Isotonic Solution

Dr. Stanley Flegler/Getty Images

• Composition of cell is similar to the isotonic solution it is in – no significant osmosis

Red Blood Cell in Hypotonic Solution

Hemolysis

David H. Phillips/Photo Researchers

• Water flows into the cell to dilute the solution inside (from the hypotonic solution)

Red Blood Cell in Hypertonic Solution

Crenation

David H. Phillips/Photo Researchers

• Water flows out of the cell to dilute the hypertonic solution outside

Colloidal Solution

oNo solute or solvent

• Contains dispersed particles that are intermediate in size between those of a solution and those of an ordinary heterogeneous mixture

• So, have dissolved solids in solvent

o FogoMilk

• A homogeneous mixture, NOT a true solution

Tyndall Effect• Colloidal Solutions scatter light (Tyndall Effect):

True Solution:

No Scattering

Colloidal Solution:

Light Scatters

Suspension

Suspension: a heterogeneous mixture containing dispersed particles that are heavy enough to settle out under the influence of gravity:

• Muddy water• Blood• Fine precipitates

Colloidal Dispersion

• Homogeneous

• Groups of small particles or individual larger molecules

• Scatters light (Tyndall effect)

• Particles do not settle

• Particles cannot be filtered out

Suspension

• Heterogeneous

• Very large particles, which are often visible

• Not transparent

• Particles settle rapidly

• Particles can be filtered out

Solutions, Colloids, and Suspensions

Dialysis• Dialysis is similar to osmosis

• Uses a semi-permeable membrane that allows the passage of solvent, dissolved ions, and small molecules, but blocks the passage of colloidal-sized particles and large molecules:

Removing Colloidal Impurities

Artificial Kidney Machine