Solutes and Solubility

Mixture Mixtures: made from physically mixing two or more substances together without a chemical reaction occurring. Mixing ionic compounds with water forms aqueous solutions of dissolved ions. The polar water molecules attract to the ions, tearing them apart from other ions and holding them away from other ions. This is called molecule-ion attraction.

Na+ Cl- O-2 H+ O-2 H+ O-2 H+ O-2 H+ O-2 H+ O-2 H+ O-2 H+ O-2 H+ O-2 H+ O-2 H+ O-2 H+ O-2 H+ O-2 H+ O-2 H+

O-2 H+ O-2 H+ O-2 H+ Na+ Cl- O-2 H+ O-2 H+ O-2 H+ O-2 H+ O-2 H+ O-2 H+ O-2 H+

O-2 H+ O-2 H+ O-2 H+ Cl- Cl- O-2 H+ O-2 H+ O-2 H+ Na+ Na+ Cl- Cl- Na+ Na+ Na+ O-2 H+ O-2 H+ O-2 H+ Cl- Cl- Na+ Na+ O-2 H+ Cl- Cl- O-2 H+ O-2 H+ Na+ Na+ O-2 H+ Na+ O-2 H+ O-2 H+ Cl- Cl- Na+ Na+

MOLECULE – ION ATTRACTION Cl- O-2 H+ O-2 H+ Na+ Partially positive ends of the water molecules (hydrogen) attracts to the negatively charged chloride ions. MOLECULE – ION ATTRACTION Partially negative ends of molecules (oxygen) attracts to the positively charged sodium ions. O-2 H+ Na+ Cl- O-2 H+

Solubility The quantity of solute that can be added to a quantity of solvent to make a saturated solution at a given temperature and pressure.

Solubility Curve Solubility Curve – a graphical representation of the amount of substance that can dissolve into 100 g of water at a specific temperature (Celsius) Substances: Compound being dissolved in water (H2O) Y-axis: Solubility of substance (g/100 g H2O) X-axis: Temperature (Celsius)

Interpreting a Solubility Curve Each point on the solubility curve shows how many grams can be dissolved at a specific temperature: Each line shows how much substance can dissolve as a function of the temperature of the solution.

Using a Solubility Curve How many grams of potassium bromide (KBr) can dissolve in 100 grams of water at 20°C? Answer: 70 grams of KBr can dissolve in 100g of water at 20°C 70g

Practice Using Solubility Curve How many grams of potassium nitrate (KNO3) can dissolve in 100 g of water at 60°C? Answer: 130 g of KNO3 can dissolve in 100 g of H2O 130g

Solution Concentration Saturated: The solution holds as many dissolved particles as it can possibly hold Unsaturated: The solution holds fewer solute particles than can theoretically be dissolved, can add more solute. Supersaturated: A very rare situation where the solution holds more solute than is theoretically possible, unstable situation where the excess will precipitate if the solution is agitated

Saturated / Unsaturated / Supersaturated Saturated: solute = solubility Unsaturated: solute < solubility Supersaturated: solute > solubility Saturated Unsaturated Supersaturated NaNO3

Factors Affecting Solubility Temperature For solid and liquid solutes, solubility in water increases as temperature increases For gaseous solutes, solubility in water decreases as temperature increases 2) Pressure For gaseous solutes, solubility increases as pressure increases Pressure does not affect solid or liquid solutes An example of this is the sugar water needed to make rock candy. A saturated solution is formed at high temperature. As the temperature cools, the sugar becomes less soluble, so water molecules holding some sugar molecules apart from each other have to jump off to assist other water molecules in keeping sugar molecules apart. The released sugar molecules re-form a crystal structure, a solid precipitate that grows slowly, that forms the rock candy crystal that is so yummy. Effect of temperature on solubility of a gas solute in water: An example of this is dissolved oxygen in a fishtank. Fish need dissolved oxygen to breathe, so it is bubbled into the tank. Gases hate being dissolved in water, because this forces their entropy to decrease, which is unfavored by nature. Increasing the temperature makes the entropy increase, which makes the gas even less likely to dissolve. At high temperatures, very little oxygen can dissolve. Tropical fish either have to have large gills to extract the small amount of dissolved oxygen from the water or have evolved to require less oxygen to survive. Cold-water fish don’t need gills that are as large, because oxygen is more soluble in colder water. In soda machines, water traveling through a tube gets flavored syrup added to it as well as carbon dioxide. Since gases hate being dissolved in water, it requires a great deal of pressure to get the carbon dioxide into the water. When a soda bottle is opened, the pressure decreases, releasing the carbon dioxide gas. If the soda bottle is left open, virtually all of the gas will escape, leaving the soda flat. CO2 gas is soluble at high pressures and nearly insoluble at low temperatures.

Factors Affecting Solubility 3) Nature of Solute and Solvent (Like Dissolves Like) Polar solutes dissolve in polar solvents Nonpolar solutes dissolve in nonpolar solvents While oil will not dissolve in water, it will dissolve very nicely in a nonpolar organic solvent called benzene. Benzene, C6H6, is as good dissolving nonpolar solutes as water is at dissolving polar solutes. The enamel in nail polish is also nonpolar, and can be nicely dissolved with another nonpolar solvent, acetone. Nail polish remover is made of acetone and nail polish itself is a solution made of enamel dissolved in acetone. When the nail polish is painted on the fingernail, the acetone evaporates, leaving the enamel behind on the nail as a precipitate. There are non-acetone nail polish removers available, they are just other nonpolar solvents.

Checking for understanding 1. Explain the difference between saturated, unsaturated, and supersaturated solutions 2. Explain how pressure and temperature affect solubility

Concentration

Concentration and molarity a measure of the amount of solute dissolved in a given quantity of solvent A concentrated solution has a large amount of solute A dilute solution has a small amount of solute

1. Grams of solute/100 mL of solvent Represents Grams of solute 100mL of solvent

2. Parts Per Million (ppm) Number of grams of solute in 1 million gram of solvent One unit of concentration used in pollution measurements that involve very low concentration g solute X 1,000,000 = ppm g solvent

A chemical analysis shows that there are 2 A chemical analysis shows that there are 2.2 mg of lead in exactly 500 g of water. Convert this measurement to parts per million. mass of solute: 2.2 mg mass of solvent: 500 g parts per million = ? g solute g solvent = ppm X 1,000,000 2.2 mg 1 g = 2.2 x 10-3 g 1000 mg 2.2 x 10-3 g X 1,000,000 = 4.4 ppm 500 g

Helium gas, 3. 0 x 10-4 g is dissolved in 200. 0g of water Helium gas, 3.0 x 10-4 g is dissolved in 200.0g of water. Express this concentration in ppm. mass of solute: 3.0 x 10-4 g mass of solvent: 200.0 g parts per million = ? g solute g solvent = ppm X 1,000,000 3.0 x 10-4 g X 1,000,000 = 1.5ppm 200 g

3. % by mass 4. % by volume g of solute % mass = x 100 % g of solution mL of solute % volume = x 100 % mL of solution Percent by volume is often used to describe the concentration of alcohol in alcoholic beverages or in medications containing alcohol.

What is the percentage by mass of a solution made by dissolving 0 What is the percentage by mass of a solution made by dissolving 0.49 g of potassium sulfate in 12.70 g of water? g of solute % mass = g of solution 0.49 g of K2SO4 = x 100 % 12.70g water + 0.49 g K2SO4 =3.7% K2SO4 by mass = 0.49 K2SO4 12.70g water

A 50.0 mL sample of an aqueous ethanol solution is distilled to yield 33.2 mL of ethanol. What is the percent by volume of ethanol in this solution? mL of solute x 100 % % volume= mL of solution 33.2 mL of ethanol = x 100 % 50.0 mL solution =66.4% ethanol by volume

5. Molarity (M) Molarity (M) is a concentration unit of a solution expressed as moles of solute dissolved per liter of solution. M = Moles of solute Liters of solution

What is the molarity of a potassium chloride solution that has a volume of 400.0 mL and contains 85.0 g KCl? 1 mol KCl solute = 85 g KCl = 1.14 mol KCl 74.55 g KCl solution = 400 mL 1 L = 0.4 L 1000 mL M = Moles L M = 1.14 mol 0.4 L = 2.85 M

An aqueous solution of sodium carbonate, Na2CO3, contains 53 g of solute in 215 mL of solution. What is it concentration (M)? solute = 53 g Na2CO3 1 mol Na2CO3 = 0.5 mol Na2CO3 105.99 g Na2CO3 solution = 215 mL 1 L = 0.215 L 1000 mL M = 0.5 mol 0.215 L M = Moles L = 2.3 M

How many moles of sugar are dissolved in 202 mL of a 0.150 M solution? solution= 202ml 1L = 0.202 L 1000ml Molarity = 0.150 M M = Moles L mol = M · L = (0.150 mol/L)(0.202 L) = 0.0303 mol C12H22O11

A mass of 98 g of sulfuric acid, H2SO4, is dissolved in water to prepare a 0.50 M solution. What is the volume of the solution in liters ? solute = 98 g H2SO4 1 mol H2SO4 = 1.0 mol H2SO4 98.08 g H2SO4 solute = 1.0mol H2SO4 Molarity = 0.50 M M = Moles L mol = M · L L = Moles M = 1.0mol 0.50 M = 2.0 L H2SO4

m = 5. Molality (m or molal) Moles of solute Kg of solvent Solute in moles and the mass of solvent in kilograms; m = Moles of solute Kg of solvent For water only! Density of water = 1 g/ 1 mL 1 mL = 1 g 1 L = 1 Kg 1000 g = 1 Kg

What is the molality of a solution with 9 What is the molality of a solution with 9.3 mole of NaCl in 450 g of water? mols of solute = 9.3 mol solution= 450g 1Kg = 0.45kg 1000g m = Moles kg 9.3 mol m = = 21 m 0.45 Kg

Determine the molality of a solution of 560 Determine the molality of a solution of 560. g of acetone, CH3COCH3, in 620 g of water. solute = 560g CH3COCH3 1 mol = 9.6 mol CH3COCH3 58.09g solution= 620g 1Kg = 0.62kg 1000g 9.6 mol m = Moles kg = = 15.5 m 0.62 Kg

Checking for understanding Formula Resulting units g of solute/100mL of solvent Parts per million % by mass % by volume Molarity Molality

Dilution

Concentration of Solution Dilution is the process of preparing a less concentrated solution from a more concentrated one. moles of solute before dilution = moles of solute after dilution MiVi = MfVf

Making a Dilute Solution remove sample moles of solute initial solution same number of moles of solute in a larger volume mix Making a Dilute Solution diluted solution Timberlake, Chemistry 7th Edition, page 344

How would you prepare 60.0 mL of 0.2 M HNO3 from a stock solution of 4.00 M HNO3? Mi = 4.00 mol/L Mf = 0.200mol/L Vf = 0.06 L Vi = ? L MiVi = MfVf Vi = MfVf Mi = (0.200mol/L ) ( 0.06L) (4.00 mol/L) = 0.003 L = 3 mL HNO3 3 mL of HNO3 + 57 mL of water = 60 mL of solution

How to mix a Standard Solution Wash bottle Volume marker (calibration mark) Weighed amount of solute Use a VOLUMETRIC FLASK to make a standard solution of known concentration Step 1> add the weighed amount of solute in the volumetric flask Step 2> add distilled water (about half of final volume) Step 3> cap volumetric flask, and shake to dissolve solute completely Step 4> add distilled water to volume marker (calibration mark) The solution process may be exothermic (release heat). This may cause the liquid to show a larger volume than is real. Allow the solution to return to ambient (room) temperature and check volume again. Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 480

Process of Making a Standard Solution from Liquids Solutions can be made using liquids or solids (or gases). To make a 5% solution v/v (volume to volume) This means to add 5 mL of solute in 95 mL of solvent. The total is 5 mL / 100 mL or 5%. For the diagram add 25 mL of liquid solute and add water to bring volume to 500 mL (about 475 mL water). SAFETY NOTE: Always add acid concentrate to water…never add water to concentrated acid. If you’ve seen what happens when water or ice crystals hit hot oil…a similar phenomenon occurs when water is added to concentrated acid. The addition of water to concentrated dissipates a large amount of heat. This heat rapidly boils the acid and causes it to spatter. If however, you start with a large volume of water and slowly add acid, the same amount of heat is generated. This time, the large volume of water is capable of absorbing the heat. The solution will not splatter. Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 483

How much of the stock solution is needed? In an experiment, a student needs 250.0 mL of a 0.100 M CuCl2 solution. A stock solution of 2.00 M CuCl2 is available. How much of the stock solution is needed? To make the solution: Pipet 12.5 mL of stock solution into a 250.0 mL volumetric flask. Carefully dilute with water to the calibration mark. Mi = 2.00 mol/L Mf = 0.100mol/L Vf = 250.0mL Vi = ? L MiVi = MfVf Vi = MfVf Mi = (0.100mol/L ) ( 2.50L) (2.00 mol/L) = 0.0125 L = 12.5 mL CuCl2

Preparing Solutions mass 45.0 g of NaCl How to prepare 500 mL of 1.54 M NaCl solution mass 45.0 g of NaCl add water until total volume is 500 mL 500 mL volumetric flask 500 mL mark 45.0 g NaCl solute

Preparing Solutions 500 mL of 1.54M NaCl molality molarity 1.54m NaCl in 0.500 kg of water 500 mL of 1.54M NaCl mass 45.0 g of NaCl add 0.500 kg of water mass 45.0 g of NaCl add water until total volume is 500 mL 500 mL water 500 mL volumetric flask 500 mL mark 45.0 g NaCl

Colligative Properties

Colligative Properties         Colligative Properties A property that depends only upon the number of solute particles (concentration), and not upon their identity. Three Important Colligative Properties of Solutions. Vapor-pressure lowering Boiling-point elevation Freezing-point depression

Vapor-Pressure Lowering Vapor pressure: is the pressure exerted by a vapor that is in dynamic equilibrium with its liquid in a closed system. A solution that contains a solute that is nonvolatile (not easily vaporized) always has a lower vapor pressure that the pure solvent. This is true because in a solution, solute particles reduce the number of free solvent particles able to escape the liquid.

Freezing-Point Depression Freezing-Point Depression: The difference in temperature between the freezing point of a solution and the freezing point of the pure solvent (water). The presence of a solute in water disrupts the formation of the orderly pattern of ice. Therefore more kinetic energy must be withdrawn from a solution than from the pure solvent to cause the solution to solidify. Solution containing solute causes decrease in freezing point.

Freezing-Point Depression

Boiling-Point Elevation Boiling Point: The temperature at which the vapor pressure of the liquid phase equals atmospheric pressure. Boiling-Point Elevation: The difference in temperature between the boiling point of a solution and the boiling point of the pure solvent.

Boiling-Point Elevation Because of the decrease in vapor pressure, additional kinetic energy must be added to raise the vapor pressure of the liquid phase of the solution to atmospheric pressure to initiate boiling. The boiling point of a solution is higher than the boiling point of the pure solvent.

Which of the following solutions will boil at the highest temperature? 1) The higher the concentration of solute is, the higher the boiling point and the lower the freezing point will be. Which of the following solutions will boil at the highest temperature? a) 100 g NaCl in 1000 g of water b) 100 g NaCl in 500 g water c) 100 g NaCl in 250 g of water d) 100 g NaCl in 125 g of water The answer is D because it has the highest concentration

2) The more particles that a solute ionizes into, the higher the boiling point and the lower the freezing point will be. Which of the following solutions will boil at the highest temperature? a) 1 mole C6H12O6 in 500 g of water b) 1 mole KBr in 500 g of water c) 1 mole MgF2 in 500 g of water d) 1 mole AlCl3 in 500 g of water The answer is D because AlCl3 breaks into 4 particles, the most of any of the choices.

Electrolytes vs. Nonelectrolytes

Electrolyte Most ionic compounds and many acids dissolve well in water. These are called electrolytes, because they cause the solution to conduct electricity due to the free-moving ions. They ionize 100% in water to yield ions in a reaction that resembles a decomposition reaction. The reaction is called dissociation, and it is a physical change, not a chemical change. The more ions a solute breaks up into, the higher the boiling point and the lower the freezing point of the solution will be.

Free moving ions will conduct electricity 2 moles of dissolved ions total Electrolytes Free moving ions will conduct electricity NaCl(s) Na+1(aq) + Cl-1(aq)

Nonelectrolytes Substances formed from covalent bonding do not dissolve into ions upon entering the water. These include polar molecules that dissolve, but do not ionize. These include sugar (C6H12O6, C12H22O11), antifreeze (CH2OHCH2OH ) and alcohol (C2H5OH). These have less impact on the melting and boiling point of a solution than ionic compounds do, because they do not break up any further.

No ions are formed, so no electricity can be conducted. Nonelectrolyte C12H22O11 (s)  C12H22O11 (aq) One mole of sucrose dissolves to form one mole of dissolved sucrose.

Checking for understanding List at least 3 characteristics for each: Electrolytes Nonelectrolytes