Concentrations & Solutions Unit 12

Mixtures Combination of 2 or more different substances Heterogeneous mixtures – can see the parts (trail mix) Homogeneous mixtures – mixture appears uniform (Kool-aid)

Types of Mixtures 1. Suspensions 2. Solutions 3. Colloids

Suspension Heterogeneous mixture Particles remain mixed with liquid when stirred but separate spontaneously over time Example – flour & water

Suspension Flour & water

Solution Homogeneous mixture with even distribution of particles Can be gas (air) or solid (brass) Most often liquid Solutes dissolved in solvent Aqueous solution – water is solvent

Solution

Colloid Stable heterogeneous mixture Appears homogeneous to naked eye b/c even distribution BUT components visible under microscope Does not settle when left to stand

Colloid

Separating Mixtures Decanting – pour off liquid

Separating Mixtures Centrifuge – spin to separate by density

Separating Mixtures Filtration – liquid through filter

Separating Mixtures Evaporation – leaves behind solid

Separating Mixtures Chromatography - based on dissolving rates

Separating Mixtures Distillation – separate by boiling point

Concentration Ratio of solute to solvent Consistent throughout solution Calculating concentration Parts per million (ppm) Molality (m) Molarity (M) g solute 1 000 000 g solvent moles solute kg solvent moles solute L solution

Molarity (M) M is read as “molar” or “moles per liter” Any amount with the same molarity will have the same concentration or ratio of solutes to solvent So 1 mL of 1 M HCl has the same concentration as 20 L of 1 M HCl

Preparing a Solution When preparing a solution, you must have the correct total volume 1.00 mole solute + 1.00 L solvent DOES NOT equal 1.00 M solution! Instead you need 1.00 L total solution So dissolve solute in small amount of solvent and then add more solvent to get the correct total volume of solution

Calculating Molarity 0.75 M KBr M = 0.30 moles KBr = 0.40 L solution 0.30 moles of KBr are dissolved in 0.40 L of solution. What is the molarity? M = 0.30 moles KBr = 0.40 L solution 0.75 M KBr

Calculating Molarity What is the molarity of a KCl solution that has a volume of 400.0 mL and contains 85.0 g of KCl? Two problems Grams mL

Molarity Calculations How many grams of CaCl2 are needed to make 625 mL of a 2.0 M solution?

Dilution Adding solvent to solutions to decrease the concentration Does not change the number of moles of the solute that are present M1V1 = M2V2

Practice 2.0 L of a 0.88 M solution are diluted to 3.8 L. What is the new molarity?

Practice You have 150 mL of 6.0 M HCl. What volume of 1.3 M HCl can you make?

Solubility The ability of a solute to dissolve in a solvent Measured in terms of the amount of a solute that will dissolve in a given amount of solvent

Solubility – “Like dissolves like” Polar substances tend to dissolve in other polar substances Nonpolar substances tend to dissolve in other nonpolar substances Degree of polarity also matters Miscible – liquids that are completely soluble in each other

Solubility – “Like dissolves like” Polar + nonpolar doesn’t usually dissolve Immiscible – 2 or more liquids that do not mix with each other

How can we speed the dissolving process? Shaking or stirring Increases the surface area contact between the solute & solvent Increasing the temperature More energy available for dissolving

Effects of temp on solubility ID trends Least soluble Most soluble Extrapolate data 200 g at 50 oC Precipitate formed from cooling

Saturation There is a maximum amount of solute that can be dissolved Saturated solution Less than that maximum amount Unsaturated solution More than that maximum amount Supersaturated solution

Colligative Properties Any physical effect of the solute on the solvent Not specific to the solute present but rather caused by the presence of a solute Examples Boiling point elevation Freezing point depression