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Concentration of Solutions

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Molarity Two solutions can contain the same compounds but be quite different because the proportions of those compounds are different. Molarity is one way to measure the concentration of a solution. moles of solute volume of solution in liters Molarity (M) =

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Mixing a Solution Procedure for preparing 0.250 L of 1.00 M solution of CuSO 4 Weight out 39.9 g of CuSO 4 = 0.250 mol Add 250 mL of water to volumetric flask Molarity =.250 mol/.250 L = 1.00 M

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Expressing the concentration of an Electrolyte When an ionic compound dissolves, the relative concentrations of the ions introduced into the solution depends on the chemical formula of the compound –Example: 1.0 M NaCl –1.0 M Na + –1.0 M Cl - 1 M Na 2 SO 4 –2.0 M Na + –1.0 M SO 4 2-

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Using Molarities in Stoichiometric Calculations

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Practice Question How many moles of HNO 3 are in 2.0 L of 0.200 M HNO 3 solution? Moles = 2.0 L soln x 0.200 mol HNO 3 / 1 L solution =0.40 mol HNO 3

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Dilution

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Solutions of lower concentration can be obtained by adding water Moles solute in conc soln = moles solute in dilution sln Therefore, M conc x V conc = M dil x V dil

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Practice problem How many liters of 3.0 M H 2 SO 4 are needed to make.450 L of 0.10 M H 2 SO 4 ? M conc x V conc = M dil x V dil Moles H 2 SO 4 in dilute solution = 0.450 L x 0.10 mol/L = 0.045 mol = moles H 2 SO 4 in conc solution L conc soln = 0.045 mol x 1 L/ 3.0 mol =.015 L

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Titration The analytical technique in which one can calculate the concentration of a solute in a solution. One reagant has known concentration

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Titration Indicators are used to determine the equivalence point of the reaction –Point where the neutralization reaction between 2 reactants are complete –Reactants in stoichiometric equivalents are brought together

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Practice Problem If 45.7 mL of 0.500 M H 2 SO 4 is required to neutralize a 20.0 mL sample of NaOH solution. What is the concentration of the NaOH solution? Moles H 2 SO 4 =.0457 L x 0.500 mol H 2 SO 4 /L = 2.28 x 10 -2 mol H 2 SO 4 Balanced Equation: H 2 SO 4 + 2 NaOH 2 H 2 O + Na 2 SO 4 Moles NaOH = 2.28 x 10 -2 mol H 2 SO 4 x 2 mol NaOH/1 mol H 2 SO 4 = 4.56 x 10 -2 mol NaOH Molarity of NaOH = 4.56 x 10 -2 mol NaOH/.020 L soln = 2.28 M NaOH

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