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Unit 8: Solutions Molarity

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Concentration Concentration is a value that represents the amount of solute dissolved in a solvent. Concentrated solutions have a large amount of solute relative to the solvent Dilute solutions have a small amount of solute relative to the solvent Concentration can be measured in Molarity, %solution, ppm (parts per million)

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Molarity Molarity is the moles of solute per liter of solution. Note the volume is the total solution volume, not the volume of solvent alone. M = moles = mass/molar mass Liters Liters This is the most common way chemists measure concentration

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How it is made… 2 Molar Solution of NaCl requires 2 moles of NaCl to be dissolved to make 1L of solution 2 moles of salt (or 117 grams) are dissolved in 1L of water to make a 2M NaCl(aq) solution

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Practice Problem #1 What is the molarity of HCl if 28g HCl is dissolved in 500.mL of solution? mass=28.gmolar mass of HCl= 36.46g/mol V= 500mL=0.500L (divide by 1000) M= moles = (mass/molar mass) Liter = (28/36.46)moles 0.500L = 1.5 M

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Practice Problem #2 How many grams of sodium nitrate (NaNO 3 ) are needed to make 2 liters of a 0.100M solution? M=0.100Mm=?mm NaNO 3 = 85g/mol 0.100M = (m/85)mols 2L (0.100M)(2L)(85)= m m=17g NaNO 3

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Solving using Dimensional Analysis Molarity can be used as a conversion factor. 2.0 L 0.100moles 85 grams = 17g 1 Liter 1 moles

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Example 3 What is the molarity of a solution with 10.0 grams of AgNO 3 is dissolved in 500.mls of solution. Molarity = Mass / Molar Mass Liters of Solution M = 10.0g / 169.88 g/mol 0.500 Liters Must convert to Liters!! 500mls/ 1000 = 0.500 Liters M = 0.118 Molar

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Example 4 How many grams of KNO 3 should be used to prepare 2.00 Liters of a 0.500M solution? Molarity = Mass / Molar Mass Liters of Solution 0.500 = mass / 101.11 2.0 (0.500)(2.0) = Mass 101.11 Mass = (0.500)(2.0)(101.11) Mass = 101.11 grams

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Example 5 To what volume should 5.0grams of KCl be diluted to in order to prepare a 0.25M solution? Molarity = Mass / Molar Mass Liters of Solution 0.25 = 5.0 / 74.55 Volume V(0.25) = 5.0/74.55 V = 0.0671 0.25 V= 0.268 Liters = 268 mLs

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