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Solutions Chapter 12

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Vocabulary Solution: a homogeneous mixture of two or more substances in a single phase Solvent: the dissolving medium in a solution Solute: the substance dissolved in a solution Solution can be liquid or solid or gaseous

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liquid-liquid solid-liquid solid-solid

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gas-liquid

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Factors Affecting the Rate of Dissolution Surface area: greater surface area speeds up the rate Agitation: shaking or stirring speeds up the rate Temperature: higher temperature usually favor a higher rate, except in the case of most gases, then a lower temperature speeds up the rate

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Like Dissolves Like Polar solutes dissolve in polar solvents Nonpolar solutes dissolve in nonpolar solvents Molecules can have a polar end and nonpolar end (especially if they are large) This is why soap will dissolve grease and then be washed away in water

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Concentration of Solutions Concentration: a measure of the amount of solute in a given amount of solvent or solution Molarity: the number of moles of solute in one liter of solution Molarity (M) = amount of solute (mol) volume of solution (L)

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Molarity One mole of NaOH has a mass of 39.998 g. If this mass is dissolved in enough water to make exactly 1.00 L of solution, the solution is a 1 M solution. If 20.0 g of NaOH is dissolved in enough water to make 1.00 L of solution, what is the molarity? 0.500 M NaOH

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Notice that 1 mol of solute added to 1 L of solvent does not usually make 1 M solution Adding the solute to 1 L will cause the volume to increase

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The preparation of a 0.0128 M solution of CuSO 4 5H 2 O Start by calculating the mass needed. Convert the mol to mass by multiplying by the molar mass. This mass is 3.20 g.

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The resulting solution has. 0.0128 mol of solute dissolved in 1.000 L of solution, which is a 0.0128 M concentration

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Problem You have 3.50 L of solution that contains 90.0 g of NaCl. What is the molarity of the solution? Given : solute mass of 90.0 g NaCl and solution volume of 3.50 L First change g to mol 90.0 g NaCl = 1.54 mol NaCl

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Continued Divide mol of solute by liters of solution 1.54 mol NaCl 3.50 L solution 0.440 M NaCl

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Problem You have 0.8 L of a 0.5 M HCl solution. How many moles of HCl does this solution contain? M = mol/L so mol = M L 0.5 M 0.8 L = 0.4 mol HCl How many grams is this? (use 3 sig figs) 14.6 g HCl

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Problem If you have 61.9 mL of a 2M solution of HCl, how many moles do you have? Convert 61.9 mL to liters by dividing by 1000. 0.0619 L mol = M L mol = 0.1238 = 0.124 mol HCl

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Dilutions Concentrated solutions are purchased in standard molarities called stock solutions You can prepare a less concentrated solution by diluting the stock solution with solvent (usually water) The number of moles of solute does not change when you dilute

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Making Dilutions The molarity times the volume of the stock solution equals the molarity times the volume of the dilution M 1 V 1 = M 2 V 2 Where M 1 V 1 is the molarity and volume of the stock solution and M 2 V 2 is the molarity and volume of the dilution

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Problem What volume, in mL of 2.00 M calcium chloride stock solution would you use to make 0.50 L of 0.300 M calcium chloride solution? Write down what you are given M 1 = 2.00 M CaCl 2 V 1 = ? M 2 = 0.300 MV 2 = 0.50 L

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Continued (2.00 M)(x) = (0.300)(0.50) 2.00x = 0.15 x = 0.075 L Since the answer has to be in mL: (0.075)(1000) 75 mL of the stock solution must be measured and then water added to make 0.50 L

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Problems What volume of a 3.00 M KI stock solution would you use to make 0.300 L of a 1.25 M KI solution? How many mL of a 5.0 M H 2 SO 4 stock solution would you need to prepare 100.0 mL of a 0.25 M H 2 SO 4 ? If you dilute 20.0 mL of a 3.5 M solution to make 100.0 mL of solution, what is the molarity of the dilute solution? 0.125 L or 125 mL 5.0 mL 0.70 M

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