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16.2 Concentration of Solutions Learning Objectives Solve problems involving the molarity of a solution Understand dilution and how to do dilution calculations Define and calculate percent by volume and percent by mass solutions Molarity (M) Molarity (M) is the most common form of expressing solution concentration Based on the total solution volume moles of solute mol M liters of solution L Molarity assumption If someone tells you that you have a 5.0 M solution you can assume this: 5 moles solute 1 liter solution Example of Molarity (M) calculation Calculate the molarity of a solution that contains 5.8g KCl in 550mL of solution. Solution: 5.8g KCl x 1 mol KCl = 0.078 mol KCl 74.55g KCl 0.078 mol KCl = 0.55 L solution Answer: 0.14M Making Dilutions Most solutions are stored as concentrated stock solutions Fresh solution made using dilution M1V1 M 2V2 Visualizing a Dilution Dilution Add Solvent Moles of solute before dilution (i) = Moles of solute after dilution (f) MiVi = MfVf Dilution Calculation How many mL of aqueous 2.00 M MgSO4 solution must be diluted with water to prepare 100.0 mL of aqueous 0.400 M MgSO4? M1= 2.00 M MgSO4 New Solution M2 = 0.400 M MgSO4 V2 = 100.0 mL of 0.400 M MgSO4 V1 = ?? mL of 2.00 M MgSO4 Stock Solution M1V1 = M2V2 Solve for V1 V1= M2V2 M1 V1=(0.400M)(100.0mL) (2.00M) V1= 20.0 mL (so 20.0 mL of stock solution will be measured and diluted with 80.0 mL of water) Percent Solutions volsolute (mL) % (v/v) 100 volsolution(mL) mass solute (g) % (m/v) 100 volsolution(mL) mass solute (g) % (m/m) 100 mass solution(g) Mass Percent % = mass solute x 100 mass solution Assumptions you can make: A 5.0% solution = 5g solute 100g solution Mass Percent Calculation What is the mass percent of 500g of water containing 6.50g of copper sulfate? Solution: 6.50 CuSO4 x 100% = (500g +6.50g) solution Answer: 1.28 % Parts per Million (ppm) Parts per million (ppm) is a ratio of parts of a solute to one million parts of a solution. Usually it is used when describing amount of contaminants in water. What is the ppm of copper sulfate if you have a solution of 500g of water containing 6.50g of copper sulfate? Solution: 6.50 CuSO4 x 106 =12833(12800 ppm) (500g + 6.50g)