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“Your Safer Source for Science Supplies” Laboratory Solution Preparation • Basic concepts of preparing solutions Laboratory Solution • Over 300 recipes of common laboratory solutions Preparation • Solution preparation tips Many of the reagents used in science are in the form of solutions in this section are available ready-made from Flinn Scientific to save which need to be purchased or prepared. For many purposes, the exact valuable laboratory prep time. value of concentration is not critical; in other cases, the concentration The section is divided into several parts for your convenience. of the solution and its method of preparation must be as accurate as possible. The Flinn Laboratory Solution Preparation reference section Basic concepts of preparing solutions is designed for both the novice and experienced solution maker. It Preparation of simple inorganic salt solutions provides valuable information on the basic concepts of preparing solu­ tions and instructions for preparing most solutions required in the high Preparations of acid and base solutions school science laboratory. Professional quality solutions are possible Recipes for Biological, Histological, and Chemical solutions when high quality and fresh chemicals and solvents are used, and meticulous procedures are followed. Many of the solutions described Basic Concepts of Preparing Solutions Molarity • Determine molarity (M1) of starting, more concentrated solu­ The most common unit of solution concentration is molarity (M). tion. The molarity of a solution is defined as the number of moles of solute per one liter of solution. Note that the unit of volume for • Calculate volume of starting solution (V1) required using equa­ molarity is liters, not milliliters or some other unit. Also note tion 2. Note: V1 must be in the same units as V2. that one liter of solution contains both the solute and the solvent. eq. 2. M1V1 = M2V2 Molarity, therefore, is a ratio between moles of solute and liters of solution. To prepare laboratory solutions, usually a given volume • Example: Prepare 100 mL of 1.0 M hydrochloric acid from and molarity are required. To determine molarity, the formula concentrated (12.1 M) hydrochloric acid. weight or molar mass of the solute is needed. The following exam­ M1V1 = M2V2 ples illustrate the calculations for preparing solutions. (12.1 M)(V1) = (1.0 M)(100 mL) V1 = 8.26 mL conc. HCl If starting with a solid, use the following procedure: Add 8.26 mL of concentrated HCl to about 50 mL of distilled • Determine the mass in grams of one mole of solute, the molar water, stir, then add water up to 100 mL. mass, MMs. Percent Solutions • Decide volume of solution required, in liters, V. Mass percent solutions are defined based on the grams of solute per 100 grams of solution. • Decide molarity of solution required, M. Example: 20 g of sodium chloride in 100 g of solution is a 20% • Calculate grams of solute (gs) required using equation 1. by mass solution. eq. 1. gs = MMs x M x V Volume percent solutions are defined as milliliters of solute per • Example: Prepare 800 mL of 2 M sodium chloride. 100 mL of solution. (MMNaCl = 58.45 g/mol) gNaCl = 58.45 g/mol x 2 mol/L x 0.8 L Example: 10 mL of ethyl alcohol plus 90 mL of H2O (making gNaCl = 93.52 g NaCl approx. 100 mL of solution) is a 10% by volume solution. Dissolve 93.52 g of NaCl in about 400 mL of distilled water, then add more water until final volume is 800 mL. Mass-volume percent solutions are also very common. These solutions are indicated by w/v% and are defined as the grams of If starting with a solution or liquid reagent: solute per 100 milliliters of solution. • When diluting more concentrated solutions, decide what Example: 1 g of phenolphthalein in 100 mL of 95% ethyl volume (V2) and molarity (M2) the final solution should be. alcohol is a 1 w/v% solution. Volume can be expressed in liters or milliliters. BASIC CONCEPTS OF PREPARING SOLUTIONS continued on next page. © 2011 Flinn Scientific, Inc. All Rights Reserved. Safety Reference 1-800-452-1261 flinnsci.com Basic Concepts of Preparing Solutions, continued Conversion Between Percent Solutions Calculating Molarity from Percent Solutions You may wish to convert mass percent to volume percent or vice To determine the molarity of a mass percent solution, versa. If so, follow this procedure: the density of the solution is required. Use the following proce­ dure: A 10% by mass solution of ethyl alcohol in water contains 10 g of ethyl alcohol and 90 g of water. 1. Determine the mass of solution by multiplying the volume of the solution by the density of the solution. 1. The formula for determining the volume of the component (ethyl alcohol in our example) is: mass = volume x density mass of ethyl alcohol 2. Determine concentration in percent by mass of the solute in Volume = —————————— density of ethyl alcohol solution. Change to the decimal equivalent. 2. Determine the volume of the total solution by dividing the 3. Calculate the molar mass of the compound, MM. mass of the solution by the density of the solution. 4. Multiply mass (step 1) by mass % (step 2) and divide by 3. Determine the percent by volume by dividing the volume of molecular mass (step 3) to find the number of moles present in the component by the volume of the solution. the whole solution. 5. Divide the number of moles (step 4) by the volume in liters of Let’s solve 1, 2, and 3 above as follows: the solution to find the molarity of the solution. 1. Mass of ethyl alcohol = 10 g (given) Example: Determine molarity of 37.2% hydrochloric acid (density 1.19 g/mL). Density of ethyl alcohol = 0.794 g/mL (from handbook) 1. Mass of solution = 1,000 mL x 1.19 g/mL = 1,190 g mass Volume = ——— density 2. Mass % = 37.2 % = 0.372 10 g Volume of ethyl alcohol = ————— = 12.6 mL 3. Molar mass of hydrochloric acid = 36.4 g/mol 0.794 g/mL 4. mass x mass % 1,190 g x 0.372 2. Mass of solution = 100 g (given) ———————— = ———————— = 12.1 moles MMHCl 36.4 g/mol Density of solution (10% ethyl alcohol) = 0.983 g/mL 5. Molarity = moles/liters = 12.1 moles/1 liter = 12.1 M (from handbook) 100 g Volume of solution = ————— = 101.8 mL* 0.983 g/mL 3. Volume percent of solution volume of ethyl alcohol 12.6 Percent = —————————— = ——— = 12.4% total volume of solution 101.8 Reverse the procedure to convert volume percent to mass percent. * The volume percent statement generally is accurate but the volume percent is not always calculated directly from the volumes of the mixed ingredients because the final volume may not equal the sum of the separate volumes. In our solution (No. Definitions 2 above) note that if the alcohol volume (12.6 mL) is added to the water volume (90 mL), the final volume is less than 102.6 mL. Buffer: A solution which tends to maintain a constant pH when excess acid or base is added. Concentrated: For some commonly used acids and bases, the maximum solubility (at room temperature) in an aqueous solution or as a pure liquid. Concentration: The relative amount of solute and solvent in a solution. Hydrates: Compounds containing water chemically combined in a definite ratio. Computations using formula weight must take the water molecules into account. Miscible: The ability of two liquids to be completely soluble in one another. Molality: A concentration unit (m); defined as the number of moles of solute divided by the number of kilograms of solvent. Molar Mass: The mass of a mole of any element or compound. Molarity: A concentration unit (M); defined as the number of moles of solute divided by liters of solution. © 2011 Flinn Scientific, Inc. All Rights Reserved. “Your Safer Source for Science Supplies” Preparation of Simple Inorganic Salt Solutions Preparation of Simple Inorganic Salt Solutions Name / Formula / F.W. Concentration g/L Name / Formula / F.W. Concentration g/L Bismuth trichloride 0.2 M 63.1 g in Aluminum chloride 0.2 M 48.3 g BiCl3 500 mL 3M AlCl3 • 6H2O 0.05 M 12.1 g 315.34 HCl* 241.43 Cadmium chloride 0.1 M 22.8 g Aluminum nitrate 0.1 M 37.5 g 1 CdCl2 • 2 ⁄2H2O Al(NO3)3 • 9H2O 228.34 375.13 Cadmium nitrate 0.1 M 30.8 g Aluminum sulfate 0.1 M 66.6 g Cd(NO3)2 • 4H2O Al2(SO4)3 • 18H2O 308.49 666.42 Calcium acetate 0.5 M 88.1 g Ammonium acetate 1.0 M 77.1 g Ca(C2H3O2)2 • H2O 0.1 M 17.6 g NH4C2H3O2 0.1 M 7.7 g 176.19 77.08 Calcium chloride 1.0 M 147.0 g Ammonium chloride 1.0 M 53.5 g CaCl2 • 2H2O 0.1 M 14.7 g NH4Cl 0.5 M 26.7 g 147.02 53.49 Calcium hydroxide saturated 2 g† Ammonium nitrate 1.0 M 80.0 g Ca(OH)2 NH4NO3 0.5 M 40.0 g 74.10 80.04 0.1 M 8.0 g Calcium nitrate 0.5 M 118.1 g Ammonium sulfate 0.1 M 13.2 g Ca(NO3)2 • 4H2O 0.1 M 23.6 g (NH4)2SO4 236.16 132.1 Chromium(III) chloride 0.1 M 26.6 g Barium chloride 0.1 M 24.4 g CrCl3 • 6H2O 266.48 BaCl2 • 2H2O 244.28 Chromium(III) nitrate 0.1 M 40.0 g Barium hydroxide 0.1 M 31.5 g Cr(NO3)3 • 9H2O 400.18 Ba(OH)2 • 8H2O 315.50 Cobalt(II) chloride 0.1 M 23.8 g CoCl • 6H O Barium nitrate 0.5 M 130.7 g 2 2 237.95 Ba(NO3)2 0.1 M 26.1 g 261.35 Cobalt(II) nitrate 0.1 M 29.1 g Co(NO3)2 • 6H2O Bismuth nitrate 0.1 M 48.5 g in 291.05 Bi(NO3)3 • 5H2O 500 mL 6M 485.1 HNO3* Copper(II) chloride 0.5 M 85.2 g CuCl2 • 2H2O 0.1 M 17.0 g 170.49 Copper(II) nitrate 0.5 M 120.8 g Normality: A concentration unit (N); defined as the number of Cu(NO3)2 • 3H2O 0.1 M 24.2 g equivalents of solute per liter of solution.
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