Chapter 3: Composition of Substances and Solutions. Some Preliminaries to Sections 3.3 and 3.4

Transcription

1 Chapter 3: Composition of Substances and Solutions Some Preliminaries to Sections 3.3 and 3.4 We are going to switch gears from looking at pure substances to studying homogenous mixtures containing water, or aqueous solutions. A solution is a homogeneous mixture composed of two or more pure substances. o The most abundant (highest percentage) component in the mixture is the solvent. o All the other components are solutes. o A solute is dissolved in the solvent, and is uniformly dispersed. In general chemistry almost all solutions we encounter are aqueous solutions in which the solvent is water. o Aqueous solutions are ubiquitous in the natural world. o Colloids and suspensions are cousins of the aqueous solution in which the solute particles are larger compared to the water molecules. o Want to expand your horizons beyond aqueous solutions? Take organic chemistry! An important property of our solutions is the concentration of each solute in the solution, that is, the amount of the solute relative to the solvent. o Concentration can be expressed qualitatively (even colloquially) or quantitatively. Qualitatively, a solute concentration can be dilute (1/2 teaspoon of sugar in a cup of coffee) or concentrated (5 teaspoons of sugar). The most important quantitative concentration for us as chemists is molarity, typically written as a number followed by a capital M. o The 0.1 M solutions in the lab are dilute, and also imprecise since concentration is expressed with only one sig fig. 1 Chem 180-Spring 2019

2 3.3 Molarity Molarity is defined as moles of solute per liter (exactly) of solution. Molarity (M)= mol solute L solution M= mol L = mol V L Calculate the molarity of a solution containing 1.75 mol of KBr in 0.75 L of solution. What is the molarity of a solution made by dissolving 2.00 g of potassium chlorate in enough water to make 150. ml of solution? The molarity equation can easily be rearranged to provide other information about the solution. o Find the moles (and optionally mass) of a solute in a given volume of solution. How many mol of sugar (sucrose) are there in 10.0 ml of a M sugar solution? How many g of sugar? 2 Chem 180-Spring 2019

3 o Find the volume of a solution containing a given number of moles of solute. What volume of a 1.50 M KBr solution contains 66.0 g KBr? You might also have to be creative with your calculations to find the concentrations of ions in solution, or to find concentrations after mixing two or more solutions. o It is very important to remember that multiple ions used to balance charge in an ionic formula dissociate (separate) when dissolved. H 2 O(l) Al 2 (SO 4 ) 3 (s) 2 Al 3+ (aq) + 3 SO 2 4 (aq) dissolving eqn So a 1 M aluminum sulfate solution is 2 M in aluminum ions and 3 M in sulfate ions. What is the molarity of chloride ions in a solution that contains 36.0 g of MgCl 2 per L of solution? Find the final molarity of the solution when 125 ml of M NaOH and 275 ml of M NaOH are mixed. Assume the volumes are additive. 3 Chem 180-Spring 2019

4 The volumetric flask is used to prepare solutions. o The flask is marked to precisely contain one volume. o Molarity is temperature dependent since the density (and hence volume) of water changes with the temperature. Dilution of a solution is achieved by adding more solvent; dilution decreases the concentration of the solution. o The dilution equation takes advantage of the fact that the moles of solute do not change. mol initial = M initial V initial mol final = M final V final and since mol initial = mol final we have M initial V initial = M final V final or more generally M 1 V 1 = M 2 V 2 the dilution eqn o If other concentration units are used, we may write C 1 V 1 = C 2 V 2 o If both initial and final volumes are in ml, we do not have to convert these to liters as the conversion factors cancel out. o A twist on this problem asks you to find the new concentration after volume is reduced by evaporation of the solvent. 4 Chem 180-Spring 2019

5 A 15.0 ml sample of M K 2 CrO 4 is diluted to ml. What is the concentration of the new solution? What volume of a M solution of glucose, C 6 H 12 O 6, can be prepared from ml of a 3.00 M glucose solution? (Personally, I don t understand why many students have difficulty with such a simple equation! Maybe you need to practice more.) When solving problems that involve molarity, it can be very helpful to rewrite M as mol solute L sol'n to help you follow and cancel the units. 3.4 Other Units for Solution Concentrations Units for other solution concentrations are usually revealed in the names. o Mass percentage mass percentage= mass of solute mass of solution 100% o Volume percentage volume percentage= volume of solute mass of solution 100% o Parts per million (ppm) and parts per billion (ppb) ppm= ppb= mass of solute mass of solution 106 ppm mass of solute mass of solution 109 ppb 5 Chem 180-Spring 2019

6 What volume of concentrated HCl solution contains 125 g of HCl? Concentrated HCl is 37.2% by mass HCl with a density of 1.19 g/ml. Wine is 12% ethanol (CH 3 CH 2 OH) by volume. Ethanol has a molar mass of g/mol and a density g/ml. How many moles of ethanol are present in a 750 ml bottle of wine? A 50.0 g sample of industrial wastewater was found to contain 0.48 mg of mercury. What is the mercury concentration of the wastewater in ppm? How much concentrated HCl(aq), in ml, is needed to prepare 12.0 L of M HCl(aq)? Concentrated HCl is 37.2% by mass HCl with a density of 1.19 g/ml. 6 Chem 180-Spring 2019