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# Final NB 2016-2017 Turley

## The Learning Goal for

The Learning Goal for this assignment is: The students will learn what properties are used to describe the nature of solutions and how to quantify the concentration of a solution Defining Concentration Measures of Concentration Concentration is defined as the amount of dissolved solute in a given amount of solvent or solution. There are several terms that describe concentration. Some of these terms are relative; that is, they can be used only to compare the concentration of one solution to another. Dilute and concentrated are two such terms. A dilute solution contains less dissolved solute than a concentrated solution (in equal volumes of solution). The terms saturated, unsaturated, and supersaturated are terms that describe concentration more precisely. Saturated: The maximum amount of solute is dissolved in a given amount of solvent at a particular temperature. Such solutions are stable. Unsaturated: Less than the maximum amount of solute is dissolved in a given amount of solvent at a particular temperature. Such solutions are stable. Supersaturated: More than the maximum amount of solute is dissolved in a given amount of solvent at a particular temperature. Such solutions are unstable. Look at the solubility curve shown below: 128 The solubility of NaNO3 is 86.0 g/100 mL H2O (at 20 °C). If you prepare a solution of 50.0 g NaNO3 dissolved in 100 mL H2O, an unsaturated solution results (Point A on the graph). continue adding NaNO to the solution until 86.0 g are dissolved, a saturated solution results (Point B). heat the solution to 50 °C, 113 grams NaNO3 can be dissolved (Point C). When the solution cools back down to 20 °C, it will be supersaturated (Point D). Quantifying Concentration To describe the concentration of a solution even more precisely, various measures of concentration can be used. Some of the ways concentration can be quantified include calculating the Mass of solute per solution mass (expressed as a percent or parts per million) Moles of solute per kilogram solvent (molality) Mass of solute per liter of solution (grams/liter) Moles of solute per liter of solution (molarity)

Part 1: Mass Percent Mass percent (also called percent by mass, weight percent, or percent by weight) compares the mass of the solute to the entire mass of the solution. Notes: 1. Determine the total mass of the sample. 2. Determine the identity of each element in the sample. 3. Determine the mass of each element in the sample. 4. Calculate the percent composition for each element: Part 2: Parts per Million Parts per million (ppm) is another measure of concentration. It is similar to mass percent. But mass percent indicates the number of grams of solute per 100 g solution. Parts per million indicates the number of grams of solute per 1,000,000 g solution. This measure of concentration is often used to express the concentrations of very dilute solutions. Notes: Parts-per notations may be expressed in terms of any unit of the same measure. Parts-per notation is often used describing dilute solutions in chemistry, for instance, the relative abundance of dissolved minerals or pollutants in water Part 3: Molality Molality (m) is the ratio of the moles of solute to the kilograms of solvent. Note: this is the first measure of concentration that is concerned with the mass of the solvent, not the mass of the solution as a whole. Notes: -a measure of the concentration of a solute in a solution in terms of amount of substance in a specified amount of mass of the solvent. -This contrasts with the definition of molarity which is based on a specified volume of solution. Part 4: Grams per Liter To express the concentration of a solution in grams per liter, you must know the mass of the solute and the volume of the solution, not just the volume of the solvent. Notes: -Grams cannot be converted directly to liters so we have a ratio -Note 1 liter= 1,000 mL Suppose you wanted to know what the concentration would be before making the solution. Could that be done? In order to relate the volume or mass of solvent to the volume of solution, you would have to know the density of the solution. You will see how solution density can be used to calculate molarity in the next section. 129

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