Chapter 16 Properties of solutions

Making solutions l A substance dissolves faster if- l It is stirred or shaken. l The particles are made smaller. l The temperature is increased. l Why?

Making solutions l In order to dissolve the solvent molecules must come in contact with the solute. l Stirring moves fresh solvent next to the solute. l Smaller pieces increase the amount of surface of the solute.

Temperature and Solutions l Higher temperature makes the molecules of the solvent move around faster and contact the solute harder and more often. l Speeds up dissolving. l Usually increases the amount that will dissolve.

How Much? l Solubility- The maximum amount of substance that will dissolve at that temperature (usually g/L). l Saturated solution- Contains the maximum amount of solid dissolved. l Unsaturated solution- Can dissolve more solute. l Supersaturated- A solution that is temporarily holding more than it can, a seed crystal will make it come out

Liquids l Miscible means that two liquids can dissolve in each other. l Immiscible means they can’t

What affects solubility? l For solids in liquids as the temperature goes up the solubility goes up. l For gases in a liquid as the temperature goes up the solubility goes down. l For gases in a liquid- as the pressure goes up the solubility goes up.

Measuring Solutions

Concentration l A measure of the amount of solute dissolved in a certain amount of solvent. l Concentrated solution has a large amount of solute. l Dilute solution has a small amount of solute l Sometimes g/l or g/mL or g/100 mL. l But chemical reactions don’t happen in grams

Molarity l The number of moles of solute in 1 Liter of the solution. l M = moles of solute /Liter of solution l What is the molarity of a solution with 2.0 moles of NaCl in 4.0 Liters of solution. l What is the molarity of a solution with 3.0 moles dissolved in 250 mL of solution. 2 mol/4liters=.5M 3 mol/.25L= 12M

Making solutions l Pour in a small amount of solvent l Then add the solute and dissolve it l Then fill to final volume. l M x L = moles l How many moles of NaCl are needed to make 6.0 L of a 0.75 M NaCl solution? l How many grams of CaCl 2 are needed to make 625 mL of a 2.0 M solution?

Making solutions l 10.3 g of NaCl are dissolved in a small amount of water then diluted to 250 mL. What is the concentration? l How many grams of sugar are needed to make 125 mL of a 0.50 M C 6 H 12 O 6 solution?

Dilution Adding water to a solution

Dilution l The number of moles of solute doesn’t change if you add more solvent. l The moles before = the moles after l M 1 x V 1 = M 2 x V 2 l M 1 and V 1 are the starting concentration and volume. l M 2 and V 2 are the ending concentration and volume. l Stock solutions are pre-made to known M

Practice l 2.0 L of a 0.88 M solution are diluted to 3.8 L. What is the new molarity? l You have 150 mL of 6.0 M HCl. What volume of 1.3 M HCl can you make? l Need 450 mL of 0.15 M NaOH. All you have available is a 2.0 M stock solution of NaOH. How do you make the required solution?

Percent solutions l Two types: Volume and Mass l Percent by volume = Volume of solute x 100% Volume of solution l indicated %(v/v) l What is the percent solution if 25 mL of CH 3 OH is diluted to 150 mL with water? 25ml/150ml*100%=17%

Percent solutions l Percent by mass = Mass of solute(g) x 100% Volume of solution(mL) l Indicated %(m/v) l More common l 4.8 g of NaCl are dissolved in 82 mL of solution. What is the percent of the solution? l How many grams of salt are there in 52 mL of a 6.3 % solution? 4.8g/82ml=5.9% 6.3/100=.063*52=3.3g

Colligative Properties Depend only on the number of dissolved particles Not an what kind of particle

Vapor Pressure l Vapor pressure is the pressure of a vapor in equilibrium with the liquid making it. l The bonds between molecules keep molecules from escaping to the vapor state. l In a solution, some of the solvent is busy keeping the solute dissolved since the surface area from which the molecules can evaporate from is reduced. l Lowers the vapor pressure. l Electrolytes form ions when dissolved - more pieces. NaCl  Na + + Cl - 2 pieces l More pieces bigger effect.

Boiling Point Elevation l Boiling point of a liquid is when the vapor pressure is equal to the atmospheric pressure. l The vapor pressure determines the boiling point. l Lower vapor pressure - higher boiling point. l Salt water boils above 100ºC l The solvent determines how much.

Freezing Point Depression l Solids form when molecules make an orderly pattern. l The solute molecules break up the orderly pattern. l Makes the freezing point lower. l Salt water freezes below 0ºC l How much depends on the solvent.

Molality l a new unit for concentration l m = Moles of solute kilogram of solvent l m = Moles of solute 1000 g of solvent l What is the molality of a solution with 9.3 mole of NaCl in 450 g of water?

Why molality? l The size of the change in boiling point is determined by the molality.  T b = K b x m x n  T b is the change in the boiling point l K b is a constant determined by the solvent(pg 523). l m is the molality of the solution. l n is the number of pieces of solute

What about Freezing? l The size of the change in freezing point is determined by the molality.  T f = K f x m x n  T f is the change in the boiling point l K f is a constant determined by the solvent(pg 524). l m is the molality of the solution. l n is the number of pieces of solute

Problems l What is the boiling point of a solution made by dissolving 1.20 moles of NaCl in 750 g of water? l What is the freezing point? l What is the boiling point of a solution made by dissolving 1.20 moles of CaCl 2 in 750 g of water? l What is the freezing point?