Objectives:• To understand the difference between a solvent

and solute• To understand the process of dissolution• To understand how polar and nonpoloar

substances dissolve• To understand Molarity in terms of solution

concentration• To understand how to calculate Molarity given

moles, volume or molarity (M)

Unit: SolutionsTopic: Molarity

Solutions

• Most of the important chemistry that keeps plants animals and humans alive takes place in aqueous solutions

• Even water that comes our of your faucet is a solution because it has dissolved minerals in it

• If you recall, a solution is a homogeneous mixture in which the components are uniformly intermingled

• Salt water is homogeneous mixture in which the first sip will be the same as the last sip

Solutions

• Solutions are not just made of liquids

• Solutions can be solids and gases as well

• Consider brass, brass is a solution made up of copper and zinc

• A gaseous solution would be air

• Air is made up of mostly nitrogen(80%) and oxygen (20%)

What is a Solution?

• a ______ mixture of ___or more substances (usually liquid)

• Ex: "he used a solution of sugar and water"

Answer BankLargest

concentrationdissolve(s)

HomogeneousLesserPolar

HigherSurfaceCrystal

TemperatureTwo

Solutions

• The substance present in the largest amount is called the solvent (usually water)

• The other substance in lesser amounts is called the solute

What is the difference between a solvent and a solute?

• The substance present in the _____amount is called the solvent (usually water)

• The other substance in ____amounts is called the solute

Answer BankLargest

concentrationdissolve(s)

HomogeneousLesserPolar

HigherSurfaceCrystal

TemperatureTwo

Practice:Complete the table by identifying the solvent and solute for each solution:

In the solution below, determine Solvent Solute

Coffee solution made up of 98.75% water and the rest consists1.25% of other chemicals

Cola is a solution made up of about 11 % sugar and roughly 88% water

Vinegar is a solution made up of 3.5 % acetic acid and 96.5% water.

Water Polarity• Remember, a water

molecule has a negative and positive dipole or charge to it

• The oxygen atom gives it a slight negative charge while the two hydrogen atoms give it a slight positive charge

• These strong charges rip and remove sodium and chlorine atoms individually

+-

+

When a compound such as salt dissolves in water we call this solvation

--

-- -

• Let’s see what happens when salt dissolves in water……

+ +

+

++

+

+

+-

-+-

-

-

Animation

• http://www.chem.iastate.edu/group/Greenbowe/sections/projectfolder/flashfiles/thermochem/solutionSalt.html

What is Solvation (dissolution)?

• The process by which a compound such as salt _____in water

• NaCl Na+1 + Cl-1Answer Bank

Largestconcentration

dissolve(s)Homogeneous

LesserPolar

HigherSurfaceCrystal

TemperatureTwo

Polar vs. NonpolarH2O = polar

electrons shared unequallyresulting in a slight positive

charge and a slight negative charge

asymetrical geometry

CH4 methane = nonpolar

electrons shared equallyNo charge

symetrical geometry

H2O = polar

electrons shared unequallyresulting in a slight positive

charge and a slight negative charge

asymetrical geometry

CH4 methane = nonpolar

electrons shared equallyNo charge

symetrical geometry

What’s the difference between polar and non-polar molecules?

Solubility of Polar Molecules• Because water is a polar

molecule it can dissolve ions such as chlorine and sodium

• But water can also dissolve other nonionic molecules such as ethanol, more commonly known as alcohol

• Like water, Ethanol has a polar O—H end to it with a positive and negative charge to it

+-

+-

+

Solubility of Polar Molecules• Ethanol dissolves in water because it is polar molecule• Polar molecules can dissolve other polar molecules

+-+

-+

Hydrogen Bond

Solubility of Polar Molecules

• Because sugar and alcohol are both polar molecules they are soluble (will dissolve in water)

• Polar molecules will dissolve in another polar liquid like water

• We say “like dissolves like”

• That is, polar molecules will dissolve other polar molecules

Substances Insoluble in Water

• But what about molecules or substances that are nonpolar?

• Why do nonpolar molecules not dissolve in in polar water molecules?

• Methane is a nonpolar molecule and has no overall charge to it. It has a symetrical geometry an the electrons are equally distributed

• Therefore, there is no attraction between the two molecules which prevent it from being soluble (dissolving)

Substances Insoluble in Water

• Many substances do not dissolve in water

• For example oil does not mix with water because it is made up of nonpolar molecules

• The C—H bond has a low negativity and the electrons are shared equally

• The result are nonpolar bonds that do no mix with the polar water molecule

How Substances Dissolve

• One rule of thumb that chemists use and worth remembering is “like dissolves like”

• In other words, polar molecules will dissolve polar molecules

• Nonpolar molecules will dissolve nonpolar molecules

What’s the difference between polar and non-polar substances?

• Polar substances will ______(soluble) in _____substances (alcohol & water)

• Non-polar substances (methane, oil) will not _____(insoluble) in ____substances (think oil and water or vinegar and water)

• “like dissolves like!”

Answer BankLargest

concentrationdissolve(s)

HomogeneousLesserPolar

HigherSurfaceCrystal

TemperatureTwo

Factors Affecting the Rate of Dissolution

• What do you think effects how fast salt will dissolve in water?

• 3 things to consider is surface area, stirring, and temperature

Factors Affecting the Rate of Dissolution

• When considering surface are, smaller salt crystals will dissolve faster than larger salt crystals

• Temperature causes the molecules to move faster increasing te rate of dissolution

• Finally, stirring removes newly dissolved particles from the solid surface and continuously exposes the surface to fresh solvent (water molecules)

What factors affect the rate of dissolution?

• 3 things:– _____ area– stirring, and – __________

Answer BankLargest

concentrationdissolve(s)

HomogeneousLesserPolar

HigherSurfaceCrystal

TemperatureTwo

Molarity

• When dealing with aqueous solutions, chemists often describe a solution in terms of concentration

• We define the concentration of a solution as the amount of solute in a given volume of solution

• The most commonly used expression of concentration when dealing with aqueous solutions is Molarity (M)

Molarity• Molarity describes the amount of solute in

moles in the volume in liters

• Molarity is describes the amount of solute in moles and volume of the solution in liters

• Molarity is define as the number of moles of solute per volume of solution in liters, or….

• M = Molarity = moles of soluteliters of solution

What is Molarity (M)?

• The number of _____ of solute per _____of solution in liters, or….

• M = Molarity =moles of soluteliters of solution

Answer Bankliters

concentratedmoles

Molarity(M)volumesolute

11.5 g NaOH

Practice:• Calculate the molarity of a solution

prepared by dissolving 11.5 grams of solid NaOH in enough water to make 1.5 Liter solution

We are given the following information:

Mass of solute = 11.5 grams

Volume of Solution = 1.5 Liters

Remember the definition of Molarity:

M = moles of solute liters of solution

Since the molarity requires mole and we are given gram, we need to convert grams to mols

M = moles of solute liters of solution

1 mol NaOH

40.0 g NaOH= 0.288 mol NaOH

Now that we have mols we can solve for MolarityM= moles of solute = 0.288 mol NaOH = 0.192 M NaOH liters of solution 1.50 L of solution

1.56 g HCl

Practice:• Calculate the Molarity of a solution

prepared by dissolving 1.56 g of gaseous HCl into enough water to make 26.8mL of solution

Remember the definition of Molarity:

M = moles of solute liters of solution

Since the molarity requires mole and we are given gram, we need to convert grams to mols

M = moles of solute liters of solution

1 mol HCl

36.5 g HCl= 0.0427 mol HCl

We are given the following information:

Mass of solute = 1.56 grams

Volume of Solution = 28.6 mL

Now that we have mol we can solve for MolarityDon’t forget to convert millileters into liters!

M= moles of solute = 0.0427mol HCl = 1.59 M HCl liters of solution 0.0268Lof solution

18 mol MgCl2

Practice:• How many grams of magnesium chloride

MgCl2 are needed to make 6.0 L of a 3.0

M solution?

Remember the definition of Molarity:

M = moles of solute liters of solution

Since we have molarity and we have liters, weTo solve for moles and then convert to grams

M = moles of solute liters of solution

95.3 g MgCl2

1 mol MgCl2

= 1,715 g mol MgCl2

We are given the following information:

Molarity of solution = 3.0 M

Volume of Solution = 6.0 L

Molarity only deal in Moles, so solving for moles we get:3.0M = mol MgCl2

6 L of solution moles MgCl2 = (3.0 M)(6.0 L) = 18 mol MgCl2

Demo:• I am going to make a 1M solution of CuSO4

• Can anyone tell how to make it?• Why don’t we make things easy.• I have a 1000ml beaker (1.0 liter)• And 159.5 grams (molar mass) which is equal

to 1 mole of CuSO4

• By dissolving 159.5 grams of CuSO4 in water, a made a 2M solution of CuSO4

Molarity (M) = 1 moles of solute = 2M 0.5 liters of solution

Summarize:

Answer Bankliters

concentratedmoles

Molarity(M)volumeSolute

OxygenNitrogenpolarity

• Gases can be solutions. Air contains 21%O2

and 79% N2, so oxygen is the solute and

___________is the solvent • In the expression like dissolves like, the word

like refers to similarity in molecular ___________.

• Molarity is equal to ______ divided _____• The more _________ a solution is, the higher

the ________• Molarity describes the amount of ____in

moles in the volume in liters