Water and Aqueous Solutions

Homogeneous Mixtures

•A mixture that has only one phase.

•Called a “solution.”

Heterogeneous Mixtures

•A mixture that has 2 or more phases

•Examples include salad dressing, muddy

water

Aqueous Solutions

•Solute: what is being dissolved

•Solvent: the medium that is doing the

dissolving

•Solution: the mixture of the solute and the

solvent

•Solvation: the process that occurs when a

solute dissolves

• Can be endo- or exo-thermic

1. Solutions

•Homogeneous mixture of two or more pure

substances.

•Mixed to the level of molecules

•Particles smaller than 1 nm

2. Suspensions

• Heterogeneous mixtures of pure substances

• Have larger particles than solutions >100nm

• Particles in a solvent are so large that they

settle out unless the mixture is constantly stirred

or agitated

3. Colloids – particles dispersed between those in

solutions and suspensions that form mixtures

•Dispersed phase – colloidal particles

•Dispersing medium – phase holding the particles

•Emulsion – special liquid–liquid colloid

• Mayonnaise!

Shows difference

between solution

and colloids

Suspended

particles catch

the light

• Electrolytes: compounds that conduct and electric

current in aqueous solution or in the molten state

• Ionic compounds such as NaCl, CuSO4, NaOH

•Don’t use a hair dryer in the bathtub!

• Nonelectrolytes: compounds that do not conduct

an electric current in the aqueous or molten state

•Covalent compounds such as sugar, alcohol, and

pure water

The ammeter measures the flow of electrons (current)

through the circuit. Phet Simulation

If the ammeter measures a current, and the bulb

glows, then the solution conducts.

If the ammeter fails to measure a current, and the bulb does

not glow, the solution is non-conducting.

Electrolytes vs. Non-electrolytes

Factors affecting the rate of dissolution

1. Increasing surface area of the solute

• Which dissolves faster: sugar cube or

granulated sugar?

2. Agitation of a solution

• Does stirring help dissolve sugar?

3. Heating a Solvent

• Which dissolves sugar faster: hot tea or iced

tea?

• Definition: physical state in

which the opposing

processes of dissolution and

crystallization of a solute

occur at equal rates

• http://phet.colorado.edu/en

/simulation/soluble-salts

• Why does salt dissolve

in water but iodine does

not?

• Why does oil NOT mix

in water?

• Why does iodine

dissolve in alcohol but

salt does not?

• “LIKE DISSOLVES LIKE”

•POLAR solutes dissolve in POLAR solutions

•NONPOLAR solutes dissolve in NONPOLAR

solutions

• WATER IS POLAR SO ONLY POLAR SUBSTANCES

WILL DISSOLVE IN IT!

• Salt is polar and dissolves in water

• Oil is nonpolar so it FLOATS on water and does

not combine with it! (salad dressing)

Saturated solution: the maximum amount of solute

dissolved in a specific amount of solvent at that

temperature

•Anywhere along the line on the graph

Unsaturated solution: less than the maximum amount

of solute dissolved in a specific amount of solvent at a

temperature

•Anywhere below the line on the graph

Supersaturated Solution:

more than the maximum

amount of solute dissolved in

a specific amount of solvent

•Usually have to increase

the temperature

•Above the line

Solids increase in solubility as the temperature

increases

•The particles have more energy and can move

faster and create more collisions

Gases decrease in solubility as the temperature

increases

•The gas particles have more energy and tend to

move out of the solution!

Why do fish die in warm weather?

• Immiscible – liquids that are

not soluble in each other

• Miscible – liquids that

dissolve freely in one

another in any proportion

Changes in pressure have very little effect on the

solubilities of liquids or solids in liquid solvents

Pressure increases the solubility of a gas in a liquid

• Henry’s Law: The solubility of a gas in a liquid is

directly proportional to the pressure of that gas on

the surface of the liquid.

𝑆1

𝑃1=

𝑆2

𝑃2

Hsoln: the amount of heat energy absorbed or

released when 1 mole of a solute dissolves a

specific amount of a solvent

Endothermic: positive heat of solution

•Feels cold!

ex) Solute + solvent + heat solution

Exothermic: negative heat of solution

•Feels hot!

ex) solute + solvent solution + heat

Endothermic

•Ammonium nitrate

•NH4NO3 (s) + H2O + Heat NH4+ (aq) + NO3

- (aq)

Exothermic

• calcium chloride or magnesium sulfate

CaCl2 (s) + H2O Ca2+ (aq) + 2 Cl- (aq) + Heat

MgSO4 (s) + H2O Mg2+ (aq) + SO4- (aq) + Heat

• The relative amount of solute in a given amount

of solvent

• Used to quantify what is in a solution

Dilute: More solvent less solute

Concentrated: More solute less solvent

•Dilute

•More solvent less solute

•Concentrated

•More solute less solvent

Concentration “[ ]” – measure of the amount of

solute in a given amount of solvent or solution

• Important for all medical & scientific fields!

Dilute: little solute

dissolved in a

solvent

Concentrated:

plenty of solute

dissolved in a

solvent

Molarity (M) – moles of solute in one liter of solution

Molarity (M) = amount of solute (mol)

liters of solution (L)

Interpret units:

• If there’s a 6 M HCl = 6 moles HCl/1 L soln

• If there’s a 0.5 M NaCl = 0.5 mol NaCl/1 L solution

1. Weigh out 1 mole of salt = 58.44 g

2. Add salt to volumetric flask

3. Add enough dH2O to fill the flask.

4. Add a stirring rod & put on stir plate.

Molarity is defined as moles of solute per liter of

solution where:

𝑀 =𝑚𝑜𝑙𝑒𝑠 𝑜𝑓 𝑠𝑜𝑙𝑢𝑡𝑒

𝐿𝑖𝑡𝑒𝑟 𝑜𝑓 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛=

𝑛

𝑉

n = moles of solute

V = volume expressed in liters

So: 𝑀 = 𝑛

𝑉 and 𝑀 × 𝑉 = 𝑛

And YES, this means you will use algebra!!

What is the molarity of a solution made by dissolving 85.0 g NaCl in

750. mL water?

Convert from grams to moles first:

85.0 𝑔 𝑁𝑎𝐶𝑙 ×1 𝑚𝑜𝑙𝑒 𝑁𝑎𝐶𝑙

58.44 𝑔 𝑁𝑎𝐶𝑙 = 1.4544 𝑚𝑜𝑙𝑒 = 1.45 𝑚𝑜𝑙 𝑁𝑎𝐶𝑙

Now solve for Molarity

𝑀 = 𝑛

𝑉 =

1.45 𝑚𝑜𝑙 𝑁𝑎𝐶𝑙

.750 𝐿 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛= 1.9333 = 1.93𝑀 𝑁𝑎𝐶𝑙

Given a concentrated solution, how do you make

a dilute solution from it?

M1V1 = M2V2

Why do we learn about this??

• In lab when we order acids they are delivered

as concentrated solutions.

• We need to calculate how to dilute them to

concentrations that we can safely use.

Example: How many liters of a 16.0 M HCl solution

are needed to make 0.500 L of a solution with a

concentration of 2.5 M HCl?

M1= 16.0 M V1= ? M2= 2.5 M V2 = 0.500 L

𝑉1 =

𝑀2𝑉2

𝑀1 =

2.5 𝑀∗0.500 𝐿

16.0 𝑀 =0.078125 L = 0.078 L

Chapter 13

Molality (m) – concentration of a soln expressed in

moles of solute per kilogram of solvent

Molality (m) = amount of solute (mol)

kilogram of solvent (kg)

Molality (m) – concentration of a soln expressed in moles of

solute per kilogram of solvent

Molality (m) = amount of solute (mol)

kilogram of solvent (kg)

Interpret units:

• If there’s a 6 m HCl = 6 moles HCl/1 kg H2O

• If there’s a 0.5 m NaCl = 0.5 mol NaCl/1 kg H2O

• 1 m HCl or 1 M HCl? Why?

• 1 m HCl has 1 kg H2O = 1 L H2O

• 1 M HCl has enough H2O to make a 1 L

soln, therefore has less water

Answer:

• 1 M HCl is more concentrated!

• A soln of iodine in carbon tetrachloride

is used when iodine is needed for

certain chemical tests. How much

iodine must be added to prepare a

0.480 m soln of iodine in CCl4 if 100.0

g of CCl4 is used?

100.0 g CCl4 x 1 kg = 0.100 kg CCl4

1000 g

0.480 m = x mol I2 0.0480 mol I2

0.1000 kg CCl4

0.0480 mol I2 x 253.80 g I2 = 12.2 g I2

1 mol I2

Ions in Aqueous Solutions and Colligative Properties

Dissociation –

separation of

ions that occurs

when an ionic

compound

dissolves

• Colligative Properties – properties that depend on the concentration of solute particles but not on their identity

• Examples

•Vapor pressure lowering which causes freezing point depression & boiling point elevation

Definition: the difference in temperature between

the freezing points of a solution and the pure

solvent

•directly proportional to the molal

concentration of the solution

•Symbol: Tf

• Molal Freezing Point Constant (Kf): the freezing-

point depression of the solvent in a 1 molal

solution of a nonvolatile, molecular solute

• Tf = Kf x m x D

• Kf of water = -1.86 oC * Kg/mol solute

• m = molal concentration (mol/kg)

• D = dissociation factor (number of particles)

Definition: the difference in temperature between

the boiling points of a solution and the pure

solvent

•directly proportional to the molal

concentration of the solution

•Symbol: Tb

Molal Boiling Point Constant (Kb): the boiling-point

elevation of the solvent in a 1 molal solution of a

nonvolatile, molecular solute

• Tb = Kb x m x D

• Kb of water = 0.51 oC * Kg/mol solute

• m = molal concentration

• D = dissociation factor (number of particles)

• What is the freezing-point depression of water

in a solution of 17.1 g of sucrose, C12H22O11, in

200. g of water? What is the actual freezing

point of the soln?

• First calculate the molality of the solution.

• Plug this into the equation to determine Tf

• Tf = Kf x m x D

• What is the freezing-point depression of water

in a solution of 17.1 g of sucrose, C12H22O11, in

200. g of water? What is the actual freezing

point of the soln?

• Tf = -1.86 0C/m x 0.25 m x 1 = -0.465 0C

MM = 342.30g/mol

0.0500 mol sucrose in .200 kg water

0.25 m sucrose

DF =1