Chapter 13

Water and Its Solutions

Objectives

• 13.1 Demonstrate the uniqueness of water as a chemical substance

• 13.1 Model the three dimensional geometry of a water molecule

• 13.1 Relate the physical properties of water to the molecular model

Objectives

• 13.2 Compare and contrast the ability of water to dissolve ionic and covalent compounds

• 13.2 Distinguish solutions from colloids • 13.2 Compare and contrast colligative

properties• 13.2 Calculate concentrations

Water

• What makes water unique– All three states of matter exist in large quantities

on earth– For its size, it has the highest melting point and

boiling point (all other molecules of similar size boil much sooner)

– It is one of few molecules which is more dense as a liquid than solid

• Most dense at 4 C

– Large specific heat

Freezing and Boiling Explained

• Water is very polar – Oxygen 2nd most

electronegative• The hydrogen atom is

an exposed proton, very attracted to other oxygen lone pairs– Why water is an acid as

well (explained later)

Hydrogen Bonding

• Occurs when Hydrogen is bound to an Oxygen or Flourine, and partially in Nitrogen

• These three (NOF) are able to expose the nucleus of the hydrogen, making it very positive and highly (electromagnetically) attractive towards other molecules

Surface Tension

• Results from imbalanced forces

• Interior molecules = Pulled in every direction

• Exterior molecules = Pulled down (but not up

Capillary

• The attraction of the molecule to the tube – Water (Blood)

likes glass more than itself, pulls itself up

– Mercury doesn’t, so it wouldn’t climb

Universal Solvent

• Water dissolves almost anything (in tiny amounts)

• Very difficult to have “pure” water because it dissolves almost anything– Hard to get clean water to people, since water

dissolves in tiny quantities the pipes

Rules of Solubility

• Likes dissolve likes– Polar dissolves Polar– Non-Polar dissolves Non-Polar

• Water = Polar, will dissolve anything with a charge – As long as charge isn’t too strong for water to pull

apart– Many salts and almost all sugars are soluble in

water

Water (polar) dissolves other polars

Sugar

Quick Vocab

• Solution: Homogenous mixture • Solute: What is being dissolved• Solvent: What is doing the dissolving• Solubility: How much of a substance can

dissolve under said conditions• Saturated: A solution which can’t dissolve any

more (Has as much as possible): Opposite- un• Supersaturated: More than normally allowed

Concentrated and Dilute

• Very vague terms• Concentrated: Associated with lots of solute• Dilute: Associated with minimal solute

Solubility

• For most salts, as temperature goes up, solubility goes up

• Sugar changes solubility a lot as well

Colligative Properties• When a pure solvent has another

element/substance dissolved into it (mixed in), the properties of the initial solvent changes.– This change is called colligative

properties

Colligative Properties• Include

– Freezing Point Depression (lower Fp)– Boiling Point Elevation (higher Bp)– Lower Vapor Pressure above solvent– Change in Osmotic Pressure

Colligative Properties• Does NOT depend on the type of

molecule dissolved• Depends on the quantity of

molecules dissolved

Colligative Properties• In order of effect per mol in water

– 1 Mol of Na3PO4 4 Ions Largest – 1 Mol of Na2SO4 3 Ions – 1 Mol of NaCl 2 Ions– 1 Mol of Sugar 1 Molecule

• For roads, place salts which dissociate into many ions

Vapor Pressure Explained

• There are less particles of solvent per Liter of solution, so the pressure from the solvent is decreased

• The amount of solute particles is directly proportional to the elevation and depression.

Colloid• A mixture, but with large particles

present suspended by charge– Note: In order to be a true solution, the

solute and solvent must have diameters of less than 1nm (1nanometer, or 1E-9m). Colloids have particles in the range of 1-1000nm

• One Å = 0.1 nanometer (Water has a diameter of 1.5 Å, so 0.15 nanometers).

• Examples include: Milk, Fog• Tyndall Effect: Scatter Light

• Blue = Water

• Yellow = Milk

SolutionsSolutions

Concentrations

Major Ideas• Expressing Concentration in a

variety of ways, including:• Molarity (M)• PPM (Parts per million)• Mass %

– Mol Fraction – Molality (m)

PPM• Often we hear the terms parts per

million and parts per billion in discussion of pollution and such.

• What does this mean? It is the number of grams of solute per million grams of solution (or billion if parts per billion)

How Safe is your water?

• Some standards for your drinking water (from EPA) (Maximum allowable levels)

• Arsenic 10ppb• Perchlorate 4ppb• Flouride 4ppm• Lead/Copper 1.3ppm

Calculating PPM• Advantage of using this method:

Useful for very small concentrations

ppXgSolution

gSolute#

Quick Question• Working for the EPA, you find

0.002g of Lead dissolved into 500.0 g of water. Is the water meeting the EPA standards for safety?– EPA = 1.3 ppm is allowable

Molarity• Describes how many mols of

solute are in each liter of solution.• Molarity = Mols / Liter of Solution

SolutionLiters

Solutemols

_

_

Molarity• 3 mols of KBr dissolved into a 0.4

L solution is 0.75 Molar (molar = molarity)

• Usefulness: Stoichiometric Calc’s

MKBrLsolution

molsKBr75.0

4.0

3.0

Review (and new)• Be careful when working with

solutions in which the solute dissociates (aqueous).

• A 1.0 Molar solution of CaCl2 is 1.0 Molar Calcium and 2.0 Molar Chloride

Questions• Determine the molarity of the

following ions. • Li+ and NO3

- in a 0.0385 M LiNO3

• Ca+2 and Cl- in a 0.035 M CaCl2• Aluminum and Sulfate in a 0.0112

M solution of Al2(SO4)3

Questions• A 3.2 Liter solution has 83grams of

Sodium Nitride (Na3N). What is the molarity of Sodium and Nitride in the solution?

Mass %• Used for many solutions you

purchasemass of A in solutiontotal mass of solutionMass % of A

= 100

Quick Question• 5 grams of acetic acid is dissolved

into 100 grams of Water. What is the mass percent of the solution?

• Your toothpaste is 0.15% flouride. If the tube has 80 grams of toothpaste, how much flouride are you buying?

Mol Fraction (Don’t need to know)

• Way of describing what % by amount of mols instead of mass

moles of Atotal moles in solution

XA =

Molality (Don’t need to know)

• Similar to Molarity, however, instead of Volume mass of solution (in kilograms)

• Useful for when temperature changes

mol of solutekg of solvent

m =

Question Check• 2.0 mols of Hydrogen Peroxide

(HOOH) is dissolved into 480grams of water. The volume of the solution 500 mL. What is the concentration of the solution in

• A) Molarity B) Mass%• C) PPM

Salinity (Don’t need to know)

• A common method for expressing the amount of salt dissolved into the ocean

• Salinity: Grams of Salt (all ions) per 1000 g of solution. The average salinity of ocean water is 35 (35 g of dissolved salt ions per 1000 g of solution).

Salinity (Don’t need to know)

• The salinity of Sodium in the ocean is 10.8 and Chlorine is 19.4. – For every 1000 grams of ocean

water, 10.8 grams of that is Sodium in its cation form

SolutionsSolutions

Dilutions

Question• 200 grams of Potassium Oxide is

dissolved into 870 grams of water and has a volume of 1.0 Liters.

• What is the • A) Molarity B) Mass % C) PPM

Diluting Solutions• Often times you will need to dilute

a more concentrated solution to a less concentrated solution. This can be done by using the equation:

• M1V1=M2V2 • Where M is Molarity and V is

Volume

Question• Example: A chemist has 2.0 L of

6.0 Molar (M) HCl (aq). • How much water will be needed to

dilute the solution to 1.0 M HCl (aq)?

Follow Up• Analysis• M1 = 6.0 M, V1 = 2.0 L, M2 = 1.0 M

(6)(2)= (1)(x)• The final volume is 12 L. However

you need to add 10 L of water (otherwise you will have a new volume of 14 L).

Which way do you go?• Always Add Acid to water. • If you add the water to the acid,

the water will be in a very small concentration, and the heat of the reaction can cause the water to boil and splatter. This splattering might have the acid with it.

Which Way Then• The proper way to dilute the

solution is by adding the 2.0 L of 6.0 M to 10.0 L of water.

Questions• Question: A chemist has 40 mL of

12.0 M HCl (aq). How much water will be needed to dilute the solution to 0.5 M HCl (aq)?

Questions• Question: A teacher needs to

make 500 mL of 2.0 M HCl for his class. The teacher has 12.0 M HCl (aq) and water. How much of each should the teacher put together to make a 500 mL 2.0 M HCl solution?

Solubility Rules• In General, what dissolves and

what doesn’t in large quantities in a water solution

Rules of Solubility• Likes dissolve likes Polar

Compounds dissolve other polar compounds

• Non-polar compounds dissolve other non-polar compounds

Solubility Rules• Vitamin B,C = Polar molecules,

dissolve into water

• Vitamin C

Solubility Rules• Vitamin C deficiency happens

quickly. Our body doesn’t store Vitamin C in an appreciable amount because it is water soluble.

• It is very hard to overdose on Vitamin C because our Kidney’s can regulate it out.

Solubility Rules• Vitamin A, D, E = Non-polar

molecules, dissolved into fat (non-polar hydrocarbon chains)

Gases and Solubility

– There is very little attraction between gases and liquid solvents. Temperature and Pressure will dictate how much gas will be dissolved.

• Temperature Increasing temperature decreases solubility

Gases: Temperature• This is because the liquid

molecules move more and the gas can sneak its way to the top.

• When Pop gets warm, it doesn’t taste as sharp, carbon dioxide escapes.

Gases: Pressure• Pressure Increasing pressure

increases solubility of gases– The gas is forced into the water

under high pressure.– When you open a can of pop, the

pressure decreases and the gas molecules can escape