Solubility, Concentrations, & Molarity

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Solubility, Concentrations, & Molarity

April 26-May 7, 2010

Solutions Definitions• Solution - homogeneous mixture consisting of

two parts

• Solvent - the substance that is dissolving the solute (present in greater amount)

• Solute - substance being dissolved

Measuring Concentration

Concentration…a measure of solute-to-solvent ratioCommon Units:

Molarity moles solute / L of solution ppm g solute / 1,000,000 g of solution

% → g solute/ 100 g of solution

concentrated vs. dilute“lots of solute” “not much solute”

“watery”

Add water to dilute a solution; boil water off to concentrate it.

Measuring Concentration“The amount of solute in a solution”

mol

L M

A. mass % = mass of solute mass of solution

B. parts per million (ppm) also, ppb and ppt – commonly used for minerals or contaminants in water supplies

C. molarity (M) = moles of solute L of solution

– used most often in this class

M = mol L

% by mass – medicated creams% by volume – rubbing alcohol

Tro's Introductory Chemistry, Chapter 13

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Mass Percent Parts of solute in every 100 parts solution.

• If a solution is 0.9% by mass, then there are 0.9 grams of solute in every 100 grams of solution.• Or 10 kg solute in every 100 kg solution.

Since masses are additive, the mass of the solution is the sum of the masses of solute and solvent.

Mass of Solute, gMass Percent 100%Mass of Solution, g

Mass of Solute Mass of Solvent Mass of Solution

Molarity

solution of Lsolute of moles(M)molarity

liter of solutionL 1mol0.25 0.25m

QUICK REVIEW OF MOLES!! The concentration can then be used to

convert the moles of solute into the liters of solution, or visa versa.

Since we normally measure the amount of solute in grams, we will need to convert between grams and moles.

How many moles are in 258 g of K2S?

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Solution Concentration Molarity

If a sugar solution concentration is 2.0 M , 1 liter of solution contains 2.0 moles of sugar, 2 liters = 4.0 moles sugar, 0.5 liters = 1.0 mole sugar:

Molarity = moles of soluteliters of solution


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Example 13.4—How Many Liters of a 0.114 M NaOH Solution Contains 1.24 mol of NaOH?

L 9.10NaOH mol 114.0L 00.1NaOH mol .241

The unit is correct, the magnitude seems reasonable as the moles of NaOH > 10x the amount in 1 L.

Check:

Solve:1.00 L solution = 0.114 mol NaOH

Solution Map:

Relationships:

1.24 mol NaOH volume, L

Given:Find:

NaOH mol 0.114nsol' L 1.00

mol NaOH L solution

Molarity

Find the molarity of a solution containing 75 g of MgCl2 in 250 ml of water.

75 g MgCl2 1 mol MgCl2

95.21 g MgCl2

= 3.2M MgCl2

0.25 L water

LmolM

Calculating Molarity

What is the molarity of a potassium chloride solution that has a volume of 400 ml and contains 85.0 grams of KCl?

Example 2

Vinegar contains 5.0 grams of acetic acid,CH3COOH, in 100.0 ml of solution. Calculate the molarity of acetic acid in vinegar.


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Practice—Determine the Mass of CaCl2

(MM = 110.98) in 1.75 L of 1.50 M Solution, Continued.Given: 1.75 L solutionFind: g CaCl2

Equivalence: 1.50 mol CaCl2 1 L solution; 110.98 g = 1 mol CaCl2

Solution Map:L solution mol CaCl2

L 1CaCl mol .501 2

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2 CaCl 291g CaCl mol 1

g 98110L 1

CaCl mol .501solution L 751 ..

Apply Solution Map:

Check Answer:Units are correct.

g CaCl2

2CaCl mol 1g 98110.

g solute / g of solution x 1,000,000=

EXAMPLEThere are 2.2 mg of lead in exactly

500g of water. Convert this to ppm.

Parts Per Million (ppm)

Assignment

Page 45 • #1, #2, #3, #4, #5, #6, #8

Answers to Page 45

2.

3.

Answers to assignment Page 45

4.

5.

8.

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Solubility When one substance (solute) dissolves in another (solvent) it is said to be

soluble.• Salt is soluble in water.

When one substance does not dissolve in another it is said to be insoluble.• Oil is insoluble in water.

There is usually a limit to the solubility of one substance in another.• Gases are always soluble in each other.• Two liquids that are mutually soluble are said to be miscible.

• Alcohol and water are miscible.• Oil and water are immiscible.

The maximum amount of solute that can be dissolved in a given amount of solvent is called solubility.


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Descriptions of Solubility Saturated solutions have the maximum amount

of solute that will dissolve in that solvent at that temperature.

Unsaturated solutions can dissolve more solute.

Supersaturated solutions are holding more solute than they should be able to at that temperature.• Unstable.

Solubility: forming a “saturated” solution in equilibrium

SATURATED SOLUTION

no more solute dissolves

UNSATURATED SOLUTIONmore solute

dissolves

SUPERSATURATED SOLUTION

becomes unstable, crystals form

increasing concentration


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Changing Temperature = Changing Solubility

When a solution is saturated, it is holding the maximum amount of solute it can at that temperature.

The solubility of the solute in the solvent depends on the temperature.

If the temperature is changed, the solubility of the solute changes.• Higher temperature = Higher solubility of solid in

liquid.• Lower temperature = Higher solubility of gas in

liquid.

Saturation Problems!!Page 44 (#1)

NaCl has a solubility of 35.9 g/100ml of water. Determine whether the following soln are sat., unsat., supersat.

• 85 g of NaCl dissolved in 150 mL of water

• 25 g of NaCl dissolved in 750 g of water

• 135 g of NaCl dissolved in 750 g of water

Saturated, Unsaturated &Super Saturated Problems

• Problems on Page 47• Do one for example• Assign other two

Assignment

Page 45-46 (concentration problems)• #9, #10, #11

Page 47 (saturation problems)• #2, #3

**DUE on Friday**

Concentration Lab

April 22, 2009

Tasks: Thursday 4-23

Complete Lab write-up from the Concentration Lab and 7th Chromatography Lab also

Page 46• #14, #15, #16, #18

10. 3.7 grams 11. 33.6 grams 16. 3.51 grams 18. 4.03 mol/ liter

Page 442. a. Unsaturated (0.789)

b. Unsaturated (0.975) c. Unsaturated (0.66)

3. a. Supersaturated (5.1 x10-4) b. Supersaturated (0.029)c. Supersaturated (7.08 x 10 -5)

Page 43 13. 189 grams

14. 0.18 L

15. 0.082L

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I Physical Properties & Characteristics of Solutions

April 24

Physical Properties of Solutions

Conductivity:• Ability to conduct an electric current • Depends on whether it contains

charged particles

Do all compounds conduct electricity?

Electrolytes

Timberlake, Chemistry 7th Edition, page 290

Electrolytes - solutions that carry an electric current

NaCl(aq) Na+ + Cl- HF(aq) H+ + F-

strong electrolyte weak electrolyte nonelectrolyte

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Electrolytes• Electrolytes are substances whose

aqueous solution (dissolved ions) is a conductor of electricity. Not all electrolytes conduct same amount of electricity.

• In strong electrolytes, all the electrolyte molecules are dissociated into ions.

• In nonelectrolytes, none of the molecules are dissociated into ions. No electric current.

• In weak electrolytes, a small percentage of the molecules are dissociated into ions.

Healthy Infoelectrolytes: solutes that dissociate in solution

-- conduct electric current because of free-moving ions e.g., acids, bases, most ionic compounds-- are crucial for many cellular processes-- obtained in a healthy diet-- For sustained exercise or a bout of the flu, sports drinks

ensure adequate electrolytes.

nonelectrolytes: solutes that DO NOT dissociate-- DO NOT conduct electric current (not enough ions)

e.g., any type of sugar


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Colligative Properties

The properties of the solution are different from the properties of the solvent.

Any property of a solution whose value depends only on the number of dissolved solute particles is called a colligative property.• It does not depend on what the solute particle

is. The freezing point, boiling point, and osmotic

pressure of a solution are colligative properties.

Ice Cream Lab Data

Average measurements over the entire day: • 6 Tbs. salt= -8*C• 12 Tbs. salt = -10*C• 18 Tbs. salt = -15*C

The more salt that was added the lower the freezing point became!

SurfactantsDefinition: A chemical agent

capable of reducing the surface tension of a liquid in which it is dissolved

Occurs at the boundary of immiscible solutions

Types: Detergent- synthetic,

works in hard water Emulsion- solution of

two immiscible substances (a colloid in which both phases are liquids; "an oil-in-water)

Soap- used to surround a molecule making it miscible (ex: surrounds oil-allows it to dissolve in water)

Cleaning Action of Soap

Micelle



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Why Do We Do That?• We spread salt on icy roads and

walkways to melt the ice.• We add antifreeze to car radiators

to prevent the water from boiling or freezing.– Antifreeze is mainly ethylene

glycol.• When we add solutes to water, it

changes the freezing point and boiling point of the water.

O2-

H+

H+H2O d-

d+d+ d-

Water MoleculeWater is a POLAR molecule

Polar water molecules hydrates Na+ and Cl- ions


HH

O

Na+

+

-- + -+

+

-

Cl-

+ -

+

hydrated ions

Dissolving of NaCl in Water

NaCl(s) + H2O Na+(aq) + Cl-(aq)

Cl-

ions

Na+

ions Water molecules


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Will It Dissolve?• Chemist’s rule of thumb:

Like dissolves like• A chemical will dissolve in a

solvent if it has a similar structure to the solvent.

• When the solvent and solute structures are similar, the solvent molecules will attract the solute particles at least as well as the solute particles to each other.

“Oil and Water Don’t Mix”

• Oil is nonpolar• Water is polar

“Like dissolves like”, nonpolar dissolves nonpolar, nonpolar does not dissolve polar

Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 470

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Will It Dissolve in Water?• Ions are attracted to polar solvents.

– Many ionic compounds dissolve in water.• Generally, if the ions total charges < 4.

• Polar molecules are attracted to polar solvents.– Table sugar, ethyl alcohol, and glucose all dissolve well in water.

• Have either multiple OH groups or little CH.• Nonpolar molecules are attracted to nonpolar solvents.

– b-carotene (C40H56) is not water soluble; it dissolves in fatty (nonpolar) tissues.

• Many molecules have both polar and nonpolar structures—whether they will dissolve in water depends on the kind, number, and location of polar and nonpolar structural features in the molecule.

Allowable Limits

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Solubility, Concentrations, & Molarity