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Aqueous Solutions

Aqueous Solutions. Concentration of Solutions. Many reactions carried out in solution - why? We can convert grams to moles and moles to grams, but… How many moles are in 15 mL of HCl(aq)? Substances dissolved in liquids

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Aqueous Solutions

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  1. Aqueous Solutions

  2. Concentration of Solutions • Many reactions carried out in solution - why? • We can convert grams to moles and moles to grams, but… • How many moles are in 15 mL of HCl(aq)? • Substances dissolved in liquids • substance is the solute, liquid is the solvent, entire mixture is the solution

  3. Molarity • Molar concentration (molarity, concentration) • number of moles of substance (solute) dissolved in one liter of solution Molarity (M) = 0.15 M HCl(aq) [HCl] = 0.15 M [HCl] = 0.15 mol/L M = n moles of solute V liters of solution there are 0.15 moles of HCl per liter of solution

  4. Molar Concentration • If you dissolved 2.5 g of KCl(s) in enough distilled water so that the volume of the solution was 75.00 mL, what is [KCl(aq)]? • M = n / V • V = 75.00 mL x 1 L = 0.07500 L 1000 mL • # mol KCl = n = 2.5 g KCl x 1 mol KCl = 0.03353 mol KCl 74.5513 g KCl • [KCl] = M = n/V = 0.03353 mol / 0.07500L = 0.45 M

  5. Molar Concentration • Suppose you want to end up with 250.0mL of a solution of NaCl(aq) • put the NaCl(s) into a volumetric flask • add some (<250.0 mL) deionized water and dissolve the NaCl(s) • finish adding water up to the 250.0mL mark on the flask • Can you instead just add 250.0mL of deionized water to the NaCl(s), then dissolve it?

  6. If you dilute an aqueous solution you add water to the solution but the number of moles of solute remains the same What changes? Volume Molarity Dilution M1V1 = n = M2V2

  7. Dilution • What volume of acid must you use to prepare 100.0 mL of 0.50 M HCl from 2.00 M HCl? • M1V1 = M2V2 • M1 = 2.00M, V1 = ?, M2 = 0.50M, V2 = 100.0 mL • V1 = M2V2 / M1 = (0.50 M)(100.0 mL) 2.00 M = 25 mL

  8. Aqueous Rxns and Chemical Analysis • We already do stoichiometry with masses • What about stoichiometry with solutions? • use M = n/V • Gravimetric analysis • usually with precipitation reactions • mixture of aqueous and solids in reaction • Volumetric analysis • often with acid-base titrations • color changes in the reaction – indicators used

  9. Gravimetric Analysis • Used with precipitation reactions (solid formed)

  10. Volumetric Analysis (Titrations) • Used with titration reactions (color produced)

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