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Concentration Calculations Molarity

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Objectives To calculate the molecular weight and moles of a substance To calculate the Molarity of a substance using the amount in moles of solute and the volume of solvent in Liters

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Calculation Review The molar weight of a molecule is calculated by adding the atomic masses for each atom in the molecule. Ex: AlCl 3 Ex2: Ca(OH) 2 = (27) + 3(35) = 132 g = 74 g = (40) + 2(16 + 1)

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Calculation Review We use moles to describe how much of a molecule we have. To calculate # moles, you need to do a t- chart and use the molecular weight. Ex: How many moles are in 140 g of Cl 2 140g Cl 2 70g Cl 2 1 mol Cl 2 MW Cl 2 = 2(35) = 70g = 2 mol Cl 2

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Calculation Review Ex: How many grams are in 0.5 moles of NaCl? MW NaCl = 23 + 35 = 58g 0.5 mol NaCl 1 mol NaCl 58 g NaCl = 29 g NaCl

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How do we know how much stuff dissolved? We can’t see the solute that has dissolved, but we can calculate how much has dissolved. There are many ways to calculate how much solute has dissolved. Molarity % by mass % by volume Parts per million (ppm) Which one you should use depends on the amount of solute, the phase of the solute, and the phase of the solvent.

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Use molarity when you have a solid solute and a liquid solvent Larger molarity values means a more concentrated solution Ex: What is the molarity of a solution that has 2 moles of KOH in 4 L of water? = 0.5 M M = 2 moles 4 L

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More Molarity Practice What is the molarity of a solution that has 37.5 g of Ca(OH) 2 in 0.25 L of water? 37.5 g Ca(OH) 2 MW Ca(OH) 2 =40 + 2(16 + 1) =74 g 74 g Ca(OH) 2 1 mol Ca(OH) 2 = 0.5 mol Ca(OH) 2 = 2 M Ca(OH) 2 M = 0.5 moles 0.25 L

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More Molarity Practice How many grams of NaCl must be used to make a 2M solution with 3 L of water? = 6 moles NaCl 2 M =x moles 3 L 6 moles NaCl 1 mole NaCl 58 g NaCl = 348 g NaCl MW NaCl =23 + 35 =58 g

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Objectives To set up and solve % by Mass and % by Volume calculations To use the density of water to convert between mass and volume of a solvent

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Solutions Calculations % By Volume % By Mass

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% Calculations You can use % by mass to describe how much solid solute has dissolved in a liquid or solid solvent. All of your numbers need to be in same unit of mass (g, kg, mg etc.) You can use % by volume to describe how much liquid solute is dissolved in a liquid solvent. All of your numbers need to be in the same unit of volume (L, mL etc.)

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Remember that the denominator has the total amount of solution, volume solvent + volume of solute! What is the concentration of 75 mL KBr in 2500 mL of water? % Vol = 2.90 %

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Be careful!! The denominator must have the total mass of the solution… g solute + g solvent! Ex: What is the concentration of a solution that has 5 g solute dissolved in 100 g solvent? % Mass = 4.76 %

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More practice… What is the concentration of 20 g KBr in 600 g of water? % Mass = 3.23 %

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1 More thing… Sometimes, the problems will assume that you know the density of water in order to calculate % mass. Density of water = 1g / 1 mL You can use the density as a conversion factor How much will 600 mL of water weigh? 600 mL 1 mL 1 g = 600 g

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% Calculation w/ Density Practice What is the concentration of 15 g HBr dissolved in 600 mL of water? 600 mL 1 mL 1 g = 600 g % Mass = 2.40 %

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More Practice How much water is needed to make a 15% solution with 5g of solute? X = 28.33 g (15)(5 + x) = 5 * 100 (15)(5 + x) = 500 15 15 5 + x = 33.33

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Solutions Calculations Parts Per Million (ppm)

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Objectives To explain what ppm or parts per million means and when to use it.

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PPM Parts per millions is a way to calculate concentrations when the amount of solute is very small.

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