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Solutions: Interpreting Chemistry

This chapter explores the concept of solutions in chemistry, including their types, solubility, composition, and concentration. It also covers important topics such as molarity and dilution.

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Solutions: Interpreting Chemistry

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

  2. 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

  3. 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 H2O attracted to anions, and vice-versa)

  4. 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 H2O • 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

  5. 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)

  6. 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

  7. 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

  8. 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

  9. 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 • M1 x V1 = M2 x V2 • Where M1 and V1 are the molarity and volume of the initial concentrated solution (before dilution) • And M2 and V2 are the molarity and volume of the final dilute solution (after dilution)

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