SOLUTIONS A mixture worth getting your hands wet in.

SOLUTIONS UNIT SKILLS Determine if a mixture is a solution Determine if a mixture is a solution Describe how various factors affect solubility and the rate of solution Describe how various factors affect solubility and the rate of solution Interpret data on solubility curves Interpret data on solubility curves Distinguish between saturated, unsaturated, supersaturated, concentrated, and dilute solutions Distinguish between saturated, unsaturated, supersaturated, concentrated, and dilute solutions Solve problems that involve concentrations of solutions Solve problems that involve concentrations of solutions Describe the relationship between solute concentration and change in the boiling and freezing points of a solvent Describe the relationship between solute concentration and change in the boiling and freezing points of a solvent

Solution Terminology Solution– homogeneous mixture Solute – substance dissolved in another substance Solvent – substance that dissolves the solute Aqueous – solution in which water is the solvent Tincture – solution in which alcohol is the solvent

Solution Characteristics Homogeneous mixtures The dissolved particles will not settle out of the solution Light can pass unobstructed through the solution Filtration will not separate out the solute Considered to be a single phase even if started as two different phases

Types of Solutions Gas Solutions – two or more gases mixed together Liquid Solutions A. Solid dissolved in a liquid B. Liquid dissolved in a liquid C. Gas dissolved in a liquid Solid Solutions – two or more solids mixed together A. Alloy – solid solution of two or more metals examples: B. Amalgam – alloy in which mercury is one of the metals

Solubility We need to find out about the process of dissolving so we can predict... Why certain substances dissolve in water.

Ionic compounds dissolving: In the case of sodium chloride, as with many other ionic compounds, when they dissolve in water, they disperse randomly and also become strong electrolytes.

Solubility is affected by the strength of the solute-solute attractions as well as the solute-solvent attractions.

Solubility of polar substances Water is able to dissolve non-ionic substances also like sugar. Also ethanol. Why? When water is a very polar molecule. We have to look at the substance itself. It must be compatible with water.

The ethanol molecule contains a polar O—H bond. Ethanol can form hydrogen bonds with water molecules.

The polar water molecule interacts strongly with the polar O—H bond in ethanol.

Do you see where the sugar molecule might be attracted to water?

Can you see any places for water to be attracted to petroleum? We know from experience that oil and water don’t mix. They are insoluble or immiscible.( 2 liquids that are mutually insoluble) “Like dissolves like”

Characteristics of Solutions Homogeneous Solute will not settle out as long as conditions aren’t changed. Light passes through Filtration will not separate One phase

Examples of types of solutions State of solution State of solute State of solvent Example Gas Air, natural gas Liquid Antifreeze in water Solid Brass LiquidGasLiquidSoda, carbonated LiquidSolidLiquidSeawater, sugar water

Solubility Amount of solute that can dissolve in a solvent Temperature – higher solubility at higher temperatures. Except gases dissolved in a liquid. Behave the opposite way. Polarity of solute and solvent. Like dissolves like. Example: vitamins Pressure – mainly affects gaseous solutes. Higher pressure more gas can dissolve.

Rate of Solution How quickly a solute dissolves Rate of Solution How quickly a solute dissolves Size of the solute particles Size of the solute particles Stirring Stirring Amount of solute already dissolved Amount of solute already dissolved Temperature Temperature

Solution Concentrations We have some very general descriptions: – Saturated: solution contains as much solute as in can at that temp. – Unsaturated: when more solute can be added and dissolves – Supersaturated: solution contains more dissolved solid than saturated solution. Very unstable – Concentrated: relatively large amount of solute present – Dilute: relatively small amount of solute is dissolved And then some more specific descriptions: – Mass Percent--Density – Molarity – Molality – Normality

Mass Percent Mass percent = Mass of solute Mass of solution x 100 Suppose a solution is prepared by dissolving 1.0 g of sodium chloride in 48 g of water. The solution has a mass of 49 g total. Find the mass percent of the solute. 1.0 g solute 49 g solution x 100 = 2.0% NaCl

Although milk is not a true solution, it does contain dissolved sugar called lactose. Cow’s milk typically contains 4.5% by mass of lactose, C 12 H 22 O 11. Calculate the mass of lactose present in 175 g of milk. Mass of solution (milk) = 175 g Mass percent of solute (lactose) = 4.5% Mass percent = Mass of solute Mass of solution x 100% 4.5% = X (unknown g solute) 175 g solution x 100% 100% 175 g X = 7.9 g solute (lactose)

Molarity The units we show as either: M M =Moles solute Liters of solution Since we deal with solutions a lot, a more convenient measure of concentration is Molarity. Or mol L

Calculate the molarity of a solution prepared by dissolving 11.5 g of solid NaOH in enough water to make 1.50 L of solution. M =Moles solute Liters of solution 11.5 g NaOH1 mol NaOH 40.0 g NaOH 1.50 L solution = M NaOH

Calculate the molarity of a solution prepared by dissolving 1.56 g gaseous HCl into enough water to make 26.8 mL of solution. M =Moles solute Liters of solution 1.56 g HCl1 mol HCl 36.5 g HCl 26.8 mL solution = 1.59 M HCl 1000 mL 1 L

Molality m =Moles solute kg solvent

Density d =Grams solute mL solution

Preparing a Standard Solution 1. Calculate # g of solute. 2. Put a little distilled water in the flask. 3. Put solute in flask, dissolve. 4. Fill to the mark.

Making a Standard Solution To analyze the alcohol content of a certain substance, a chemist needs 1.00 L of an aqueous M K 2 Cr 2 O 7 solution. Give directions to mix L soln0.200 mol K 2 Cr 2 O 7 1 L g K 2 Cr 2 O 7 1 mol =58.8 g K 2 Cr 2 O 7 3. Put K 2 Cr 2 O 7 in flask, dissolve. 2. Put small amount of distilled water in 1 L flask. 1. Weigh out 58.8 g K 2 Cr 2 O 7 4. Fill to the mark with distilled water.

Dilution Sometimes, concentrated solutions are purchased. Water is added to achieve the desired molarity for a particular solution. M 1 V 1 = M 2 V 2 Prepare 500. mL of 1.00 M acetic acid (HC 2 H 3 O 2 ) from a 17.5 M stock solution. (17.5 M) V1V1 =(1.00 M)(500 mL) V 1 = 28.6 mL

How many moles of Ag + ions are present in.025 L of a 0.75 M AgNO 3 solution?.025 L1 mol Ag + ions 1 mol AgNO 3 =.019 mol Ag mol AgNO 3 1 L Remember: 1 mole of AgNO 3 = 1 mole Ag + ions

Properties of Solutions: boiling & freezing points What does adding NaCl to a pot of boiling water do? Why do we spread “ice melt” on our sidewalks in the winter? Why do we add “antifreeze” to our car radiators? Adding a solute to a solvent changes the vapor pressure of the solution....thus changing the boiling and freezing points!

Boiling in a pure solvent Those solvent molecules in the bubble have enough energy to break all intermolecular attractions

When a solute is added: It may be argued that the solute reduces the number of solvent molecules that can get into the bubble. Thus it takes more energy (heat) to “boil” the solvent....this is the “boiling point elevation”

Freezing point is also affect by the addition of a solute. At the point of freezing & melting, the vapor pressures of the solid and liquid are at equilibrium. Since adding a solute lowers the vapor pressure of a substance, the vapor pressure of a liquid is equal to that of the solid at a lower “temperature”. These properties— boiling point freezing point vapor pressure osmotic pressure of solvent --are called Colligative Properties

Freezing Point Depression Normal freezing point occurs at a temperature where the vapor pressure of the solid and liquid are equal to each other.  T f = K f m solute Kf is the freezing point constant for the solvent. Normal freezing point occurs at a temperature where the vapor pressure of the solid and liquid are equal to each other. The solid does not form until the temperature is lower than in the pure solvent. We say that a non-volatile solute depresses the freezing point of a solution and can calculate it with: The solid does not form until the temperature is lower than in the pure solvent.

11.9 What mass of ethylene glycol (C 2 H 6 O 2 ) must be added to 10.0 L water to produce a solution for use in a car’s radiator that freezes at o F (-23.3 o C)? Assume the density of water is exactly 1 g/mL.  T f = K f m solute  T = 23.3 o C K f = 1.86 o C kg/mol 10.0 kg solvent used 23.3 o C = (1.86 o C  kg/mol) m solute m solute =12.53 mol C 2 H 6 O 2 kg solvent 10 kg water = mol C 2 H 6 O mol C 2 H 6 O 2 = 7769 g mol 1.86 o C  kg 62 g C 2 H 6 O 2 1 mole

Osmosis A semi-permeable membrane separates a solution and pure solvent—allowing the solvent but not the solute to pass. Osmosis is the flow of solvent to the solution until the solution reaches equilibrium Osmotic pressure is the hydrostatic pressure on the solution and depends on the concentrations….