Water and Aqueous SystemsChapter 15

Properties of Water

• High surface tension• Low vapor pressure• High boiling point

**All due to high intermolecular forces between water molecules due to HYDROGEN BONDING!

Surface Tension

• Surface tension is the inward force, or pull, that tends to minimize the surface area of the liquid

• A surfactant can interfere with the hydrogen bonding of water molecules and reduce surface tension

How does a lake freeze and not kill all the fish?

HOMOGENEOUS AQUEOUS SYSTEMS

What are Solutions?

• Solution : a uniform (homogeneous) mixture that may contain solids, liquids or gases.

• Parts of a solution:– Solute: What gets dissolved in a solution – usually

present in smaller amounts– Solvent: What does the dissolving in a solution –

usually present in larger amounts – WATER is the “UNIVERSAL SOLVENT”

Dissolving Process“Solvation”

• http://www.northland.cc.mn.us/biology/Biology1111/animations/dissolve.html

• This animation shows how the intermolecular forces between the ions and the polar water molecules overcome the ionic forces holding Sodium and Chloride ions together

HETEROGENEOUS AQUEOUS SYSTEMS

Suspensions

• Suspensions have particles that are much larger than those in solutions AND the particles will eventually settle out

Colloids

• Colloids have particles smaller than those in suspensions and larger than those in solutions

• The particles of a colloid do not settle out• The Tyndall Effect states that visible light can

be scattered by colloidal/suspended particles

Solutions

Chapter 16

Some terms associated with solutions

• Soluble: a substance that dissolves in a solvent is said to be “soluble” in that solvent

• Insoluble: a substance that does not dissolve in a solvent is said to be “insoluble” in a solvent

• Miscible: two substances are able to be mixed (think “mixable”)

• Immiscible: two substances are not able to be mixed (think “not mixable”)

More solution terms• Aqueous : a solution with water

as the solvent (something dissolved in water)

• Solubility : refers to the maximum amount of solute that will dissolve in a given amount of solvent at a specified temperature and pressure

Types of Solutions

• Saturated solution: contains the maximum amount of dissolved solute for a given amount of solvent at the specified temperature and pressure

• Unsaturated solution: contains less dissolved solute for a given temperature and pressure than a saturated solution (could still dissolve more)

• Supersaturated solution: contains more dissolved solute than a saturated solution (made by increasing temperature and/or pressure to dissolve and then cooled slowly)

Solution Concentration (cont’d)

• A “Concentrated” solution contains a large amount of solute relative to its solubility

• A “dilute” solution contains a small amount of solute relative to its solubility

Factors Affecting Solubility• Solvation = process of surrounding solute

particles with solvent particles to form a solution– Ionic Compounds – charged ends of the water

molecule attract the positive and negative ions of the ionic compound, breaking them apart and surrounding them

– Molecular compounds – each O-H bond in the molecule becomes a site for hydrogen bonding with the attractive forces between polar water molecules and other polar molecules

• “LIKES DISSOLVE LIKES” – Polar solvents dissolve polar solutes & nonpolar solvents dissolve nonpolar solutes

Factors Affecting Solubility

• Agitation stirring or shaking increases rate of solution

• Surface area larger surface area (smaller particles) increases rate of solution

• Temperature higher temperature (more kinetic energy and collisions) increases rate of solution

Henry’s Law

• States that at a given temperature, the solubility (S) of a gas in a liquid is directly proportional to the pressure (P) of the gas above the liquid

S1 S2=

P1 P2• Solubility units will be mass/volume

– g/L or mg/ml

Henry’s Law Problem

• If 0.85 g of a gas at 4.0 atm of pressure dissolves in 1.0 L of water at 25°C, how much will dissolve in 1.0 L of water at 1.0 atm of pressure and the same temperature?

Units of Concentration

Molarity• Concentration of a solution.

solution of liters

solute of moles(M)Molarity

total combined volume

substance being dissolved

Molarity

2M HCl

L

molM

L 1

HCl mol 2HCl 2M

What does this mean?

Molarity Calculationsmolar mass

(g/mol)

6.02 1023

(particles/mol)

Molarity(mol/L)

Molarity Calculations• How many grams of NaCl are

required to make 0.500L of 0.25M NaCl?

0.500 L 0.25 mol

1 L

= 7.3 g NaCl

58.44 g

1 mol

L 1

mol0.25 0.25M

Molarity Calculations• Find the molarity of a 250 mL

solution containing 10.0 g of NaF.

10.0 g 1 mol

41.99 g = 0.238 mol NaF

0.238 mol

0.25 L M = = 0.95M NaF

L

molM

The Dilution Equation

M1V1 = M2V2

M1 = initial molarity (“stock solution”)

V1 = initial volume (Liters)

M2 = final (desired) molarity

V2 = final volume (Liters)

This equation is used when you have a “stock solution” of higher molarity than you need and you need to dilute it to a lower molarity by adding additional solvent.

Dilution Equation (cont’d)

What volume of 3.00M KI stock solution would you use to make 0.300 L of a 1.25M KI solution?

Other Concentration Units

Percent by Mass

mass of solutePercent by mass = ___________________ x 100% mass of solution

In order to maintain a sodium chloride (NaCl) concentration similar to ocean water, an aquarium must contain 3.6 g NaCl per 100.0g of water. What is the percent by mass of NaCl in the solution?

Percent by Volume

volume of solutePercent by volume = _______________ x 100% volume of solution

What is the percent by volume of ethanol in a solution that contains 35 mL of ethanol dissolved in 155 mL of water?

Molality

moles of soluteMolality (m) = ________________ kg of solvent

In the lab, a student adds 4.5 g of sodium chloride (NaCl) to 100.0 g of water. Calculate the molality of the solution.

Mole Fraction

• A mole fraction equals the number of moles of solute or solvent in a solution divided by the total number of moles of solute and solvent

XA = nA

nA + nB

XA = mole fraction of substance A

nA = moles of A

nB = moles of B

Mole Fraction Example

• What is the mole fraction of NaOH in an aqueous solution that contains 22.8% NaOH by mass?

Enough Already!

Let’s stop here for today…

Colligative Properties

• Colligative properties depend on the number of solute particles in a solution, not on the identity of the solute particles

– Vapor Pressure Lowering– Boiling point elevation– Freezing point depression– Osmotic pressure

Vapor Pressure Lowering

Vapor pressure is the pressure exerted in a closed container by liquid particles that have escaped the liquid’s surface and entered the gaseous state.

• With fewer solvent particles (more solute particles) at the surface, fewer particles enter the gaseous state, and the vapor pressure is lowered

Raoult’s Law *out of order*

Psoln = XsolventP0solvent

Where:Psoln = observed vapor pressure of the solution

Xsolvent = mole fraction of the solvent

P0solvent = vapor pressure of the pure solvent

Raoult’s Law Problem*some changes were made to slide• Calculate the expected vapor pressure at 25°C

for a solution prepared by dissolving 158.0 g of sucrose (molar mass = 342.3 g/mol) in 641.6 g of water. At 25°C, the vapor pressure of pure water is 23.76 torr.

Boiling Point Elevation

A liquid boils when its vapor pressure equals the atmospheric pressure.

• More solute particles in solution cause a lower vapor pressure and thus it takes a higher temperature to reach boiling.

Calculating BP Elevation

ΔTb = Kbm

Where:ΔTb = bp elevation

Kb = molal boiling point elevation constant (will be given to you)

m = molality (either given to you, or may need to calculate this)

Freezing Point Depression

The freezing point of a solution is always lower than that of a pure solvent.

• At a solvent’s freezing point, the particles no longer have enough kinetic energy to overcome intermolecular forces and the particles form into an ordered solid.

• In a solution, the solute particles interfere with the intermolecular forces between solvent particles and the temperature needs to be even lower in order to freeze and become solid

Calculating FP Depression

ΔTf = Kfm

Where:ΔTf = fp depression

Kf = molal freezing point depression constant (will be given to you)

m = molality (may need to calculate this!)

Sample calculation

• What are the boiling point and freezing point of a 0.40m solution of sucrose in ethanol?

For Ethanol, Kb = 1.22 °C/m, Kf = 1.99 °C/m, boiling point = 78°C, freezing point = -114°C

Osmotic pressure

Osmosis is the diffusion of a solvent through a semi-permeable membrane (allows some particles through and not others)

• During osmosis, water molecules can move in both directions (from dilute to concentrated) across the membrane, but the solute particles cannot

• Osmotic pressure is the amount of additional pressure caused by the water molecules that moved in to the concentrated solution

Determining Molar Mass from Osmotic Pressure

Π = MRT

Where:Π = osmotic pressure in atmospheresM = molarity of solutionR = universal gas law constant (0.08206 L*atm/mol*K)T = temperature in Kelvin

Determining Molar Mass from Osmotic Pressure

• To determine the molar mass of a certain protein, 1.00 g is dissolved in enough water to make 1.00 L of solution. The osmotic pressure of this solution was found to be 0.00147 atm at 298 K. Calculate the molar mass of the protein.