Concentrations & Solutions

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 0.30 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

Concentrations & Solutions