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Friday, Feb. 21 st : “A” Day Monday, Feb. 24 th : “B” Day Agenda Collect chromatography labs Begin Section 13.2: “Concentration and Molarity” Demos: Preparing 250 mL of a 0.5000 M CuSO 4 solution Do solvents always add up? Homework: Practice pg. 461: #2, 3, 5, 6 Practice pg. 465: #1-7

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Concentration In a solution, the solute is distributed evenly throughout the solvent. This means that any part of a solution has the same ratio of solute to solvent as any other part of the solution. This ratio is the concentration of the solution. Concentration: the amount of a particular substance in a given quantity of a solution

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Calculating Concentration Concentrations can be expressed in many forms.

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Calculating Concentrations One unit of concentration used in pollution measurements that involve very low concentrations is parts per million, or ppm. Parts per million is the number of grams of solute in 1 million grams of solution.

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Sample Problem A, Pg. 461 A chemical analysis shows that there are 2.2 mg of lead in exactly 500 g of a water sample. Convert this measurement to parts per million. First, change mg g: 2.2 mg Pb X 1 g =.0022 g Pb 1,000 mg Divide by 500 g to get the amount of Pb in 1 g of H 2 O. Then multiply by 1,000,000 to get the amount of Pb in 1,000,000 g H 2 O:.0022 g Pb X 1,000,000 parts = 4.4 ppm Pb 500 g 1 million (2 sig figs)

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Additional Example How many parts per million of mercury are there in a sample of tap water with a mass of 750 g containing 2.2 mg of Hg? First, change mg g: 2.2 mg Hg X 1 g =.0022 g Hg 1,000 mg Divide by 750 g to get the amount of Hg in 1 g of H 2 O. Then multiply by 1,000,000 to get the amount of Hg in 1,000,000 g H 2 O:.0022 g X 1,000,000 parts = 2.9 ppm Hg 750 g 1 million (2 sig figs)

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Molarity Since the mole is the unit chemists use to measure the number of particles, they often specify concentrations using molarity. Molarity (M): a concentration unit of a solution expressed as moles of solute dissolved per liter of solution. Molarity (M) = moles of solute L of solution

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Molarity Example Suppose that 0.30 moles of KBr are present in 0.40 L of solution. The molarity of the solution is calculated as follows: 0.30 mol KBr= 0.75 M KBr 0.40 L solution This is called a 0.75 molar solution of KBr.

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Preparing a Solution of a Specified Molarity Note that molarity describes concentration in terms of volume of solution, NOT volume of solvent. If you simply added 1.000 mol solute to 1.000 L solvent, the solution would not be 1.000 M. The added solute would increase the volume, so the solution would not have a concentration of 1.000 M. The solution must be made to have exactly the specified volume of solution.

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Demo: Preparing 250 mL of a 0.5000 M CuSO 4 Solution (pg. 463)

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Calculating Molarity In working with solutions in chemistry, you will find that numerical calculations often involve molarity. The key to all such calculations is the definition of molarity… Molarity (M) = moles of solute L of solution

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Calculating Molarity Given Mass of Solute and Volume of Solution

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Sample Problem B, Pg. 465 What is the molarity of a potassium chloride solution that has a volume of 400.00 mL and contains 85.0 g KCl? Molarity = moles of solute L of solution First, use molar mass to change g of KCl → moles KCl: 85.0 g KCl X 1 mol KCl = 1.14 mol KCl 74.6 g KCl 1.14 mol KCl = 2.85 M KCl.400 L (3 sig figs)

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Additional Example Determine the molarity of a solution prepared by dissolving 16.9 g of NaOH in enough water to make 250.00 mL of solution. Molarity = moles of solute L of solution First, use molar mass to change g NaOH mol NaOH 16.9 g NaOH X 1 mol NaOH = 0.423 mol NaOH 40 g NaOH 0.423 mol NaOH = 1.69 M NaOH 0.250 L (3 sig figs)

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Calculating Mass of Solute Given Molarity and Volume of Solution

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Additional Example How many grams of NaOH are needed to prepare 250.0 mL of a 1.69 M NaOH solution? First, change mL L: 250 mL X 1 L = 0.2500 L 1,000 mL Then, multiply by molarity to find moles of solute: 0.2500 L X 1.69 moles NaOH = 0.423 mol NaOH 1 L Finally, use molar mass to find mass of solute: 0.423 mol NaOH X 40 g NaOH = 16.9 g NaOH 1 mole NaOH (3 sig figs)

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Additional Example How many grams of glucose, C 6 H 12 O 6, are in 255 mL of a 3.55 M solution? First, change mL L: 255 mL X 1 L = 0.255 L 1,000 mL Then, multiply by molarity to find moles of solute: 0.255 L X 3.55 moles glucose = 0.905 mol 1 L glucose Finally, use molar mass to find mass of solute: 0.905 mol glucose X 180 g glucose = 163 g glucose 1 mol glucose

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Demo: Do Solvents Always Add Up? 50.0 mL H 2 O + 50.0 mL ethanol = ________?___ mL solution +

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Homework Practice pg. 461: # 2, 3, 5, 6 Practice pg. 465: # 1-7 We will finish section 13.2 next time…

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