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CHEM 212
Elaine Power

Solvent determines whether the solution exists as a solid, liquid, or a gas. However, for learning purposes, the aqueous (aq) solution will be used as a solvent. Solute is the material being dissolved by the solvent. Aqueous reactions can be grouped into three general categories: Precipitation reactions, acid-base neutralization reactions and oxidation-reduction (red ox) reactions. Molarity Molarity is a solution property defined as the number of moles solute per liter of solution Molarity (M) = (amount of solute, in moles / volume of solution, in liters) The term M stands for the term molar or mol/L. Problem: A solution is prepared by dissolving 25.0 mL ethanol, C H OH (d= 0.789 g/mL), in enough water 2 5 to produce 250.0mL solution. What is the molarity of ethanol in the solution? Solution: To determine the molarity, we must first find how many moles of ethanol exists in a 25.0 mL sample. ? mol C H2OH5= [25.0 mL C H OH x2( 5.789g C H OH/ 1 mL 2 H5OH) x ( 1 mol 2 5 OH/ 46.07 g C H O2)]5 2 5 = 0.428 mol C H O2 5 Now apply the definition of molarity to this answer. Thus divide the moles of C H OH by the solution 2 5 size, 0.2500L. Molarity = (0.428 mol C H OH 2 5.2500 L soln) = 1.71M C H OH 2 5 Preparation of a Solution 1. Weigh the solid sample. 2. Dissolve it in a volumetric flask partially filled with solvent. 3. Carefully fill to the mark. Solution Dilution M i V = i x V f f Problem: A particular analytical chemistry procedure requires 0.0100 M K CrO . What volume2of 04250 M K C2O mu4t be diluted with water to prepare 0.250L of 0.0100 M K CrO ? 2 4 Solution: ? mol K C2O = [4.250 L soln x (0.0100 mol K CrO /1L s2ln)] 4 0.00250 mol K CrO 2 4 ? L soln = [0.00250 L soln x ( 1L soln / 0.250 K CrO ] 2 0.0400 L soln OR V i V xfM/M =f[25i.0mL x (0.0100 M/ 0.250 M)] = 10.0 mL Determining Limiting Reactant The reactant that is completely consumed is called the limiting reactant. The reactant that remains after a reaction is done is said to be in excess. Problem: Phosphorus trichloride, PCl , is a3commercially important compound used in the manufacture of pesticides, gasoline additives, and a number of other products. It is made by the direct combination of phosphorus and chlorine. P + 6 Cl → 4 PCl 4(s) 2(g) 3(l) What mass of PCl 3 (l)orms in the reaction of 125g P with4323g Cl ? 2 Solution:
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