Chapter 14: Solutions Chemistry 1020: Interpretive chemistry Andy Aspaas, Instructor

Solutions Solution = homogeneous mixture –Components of mixture are equally dispersed throughout the mixture Solutions can be solid, liquid, or gas –Air = solutions of gases –Brass = solution of copper and zinc Solvent: component of solution that is present in the largest amount Solutes: other components of a solution Aqueous solutions: water is solvent

Solubility Both sugar and salt readily dissolve in water –In these cases, a solid solute (salt or sugar) dissolves in the liquid solvent (water) Ionic solids like NaCl consist of closely packed ions which are held together by strong ionic bonds –But, when dissolved in water, the polar water molecules are attracted to the ions, and can overcome ionic bonds –The ions are separated and surrounded by water molecules (  - of H 2 O attracted to anions, and vice-versa)

Solubility and molecular substances Most water-soluble molecules have some type of hydrogen-bonding source and/or polar bonds –Ethanol contains a polar OH group which can hydrogen-bond with water –Sucrose (sugar) contains many OH groups –  + on H of OH groups attracted to  — on O of H 2 O Nonpolar molecules like petroleum have no polar OH groups and are not able to form hydrogen bonds with water; they are insoluble –Petroleum forms a layer on top of water

Like dissolves like Like dissolves like: a solvent will usually dissolve solutes with polarities similar to its own Polar solvents like water will dissolve polar molecules Nonpolar solvents like hydrocarbons will dissolve nonpolar solutes –Drycleaners use nonpolar solvents to remove grease stains (greases are composed of very nonpolar molecules)

Solution composition There is a limit to how much solute a particular solvent can dissolve A saturated solution contains as much solute as will dissolve at that particular temperature An unsaturated solution has not yet reached that limit A solution with a relatively high amount of solute is called concentrated A solution with a relatively low amount of solute is called dilute

Mass percent in solutions Mass percent (sometimes called weight percent) [ (grams solute) / (grams solution) ] x 100% Often given in nonscientific applications Vinegar is approximately 5% by mass acetic acid in water –That means 100 g of vinegar would contain 5 g acetic acid When calculating mass %, don’t forget to include solute mass in the total solution mass Solution mass = solvent mass + solute mass

Molarity Concentration: amount of solute in a given volume of solution (often more convenient than dealing with masses of solutions) Molarity, M, is most common excpression of concentration M = (moles solute) / (liters solution) = mol / L A 1.0 molar solution (written 1.0 M) contains 1.0 mol solute per liter of solution

Dilution In a dilution, water is added to a concentrated solution in order to make it more dilute The number of moles of solute is equal before and after the dilution since only water is added But the concentration has changed M 1 x V 1 = M 2 x V 2 –Where M 1 and V 1 are the molarity and volume of the initial concentrated solution (before dilution) –And M 2 and V 2 are the molarity and volume of the final dilute solution (after dilution)