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4/15/2015 CONCENTRATION • In chemistry we often refer to solutions as dilutes or concentrated CONCENTRATION OF SOLUTIONS • Refers to the amount of solute involved • Express concentration using different calculations MOLARITY EXAMPLE • Number of moles of solute per liter of solutions • What is the molairty of a solution that contains 4.0 mol of NaOH in .50L of solution? • To calculate use the formula • Substitute in the formula • Molarity = Moles of Solute Liters of solution • Molarity = Moles of Solute Liters of solution Molarity = 4.0 moles NaOH .50 liter • M = 8.0 1 4/15/2015 EXAMPLE • What is the molarity of a solution containing 82.0 g of Ca(NO3)2 in two liters of solution? • Step 1 Covert grams to moles • GFM= 164 g/m of Ca(NO3)2 • Moles= 82.0 164 = .500 mols CONT……. • Step 2 Calculate molarity • M = .500 mol 2.0 liters M = .250 REMINDER • Answers need to be calculated using liters • If a question uses ml instead of liters divide by 1,000 • Ex. 500 ml = 0.5 L TRY • Find the molarity of 100 mL of a solution that contains 0.25 moles of dissolved solute • M = .25 .100 2.5M 2 4/15/2015 PARTS PER MILLION PARTS PER MILLION • Is the ratio between the mass of a solute and the total mass of the solutions • This is used when the concentrations are low or diluted (difficult to calculate) • Ppm = grams of solute x 1,000,000 grams of solution • Ex. Swimming pools • 2 grams of Cl per 1,000,000 grams of water • Or particles in the air EXAMPLE • Approximately .0043 g of oxygen can be dissolved in 100.00ml of water at 20 degrees. Express this in parts per million EXAMPLE • Ppm = .0043 g x 1,000,000 100.0043 g Ppm = 43 • Grams of solute = .0043 g • Grams of solution = 100.0043 g 3 4/15/2015 TRY CHALLENGE QUESTION • What is the ppm of a solution containing .008 g NaOH dissolved in 1000 grams of H2O? • How many grams of NaCl are needed to be dissolved in water to make 100 grams of a 250 ppm solution? • Answer = .025g • Ppm = .008g x 1,000,000 1000.008 • 250 = x_ x 1,000,000 100 • Divide 250/1,000,000 =.00025 • Multiply .00025*100 = 8 ppm PERCENT BY MASS • Is the mass of an ingredient divided by the total mass as a percent • %mass = Mass of part x 100 mass of whole EXAMPLE • What is the percent mass of NaOH if 2.50g of NaOH are added to 50.00 grams of H2O? • Use the formula • %mass = Mass of part x 100 mass of whole 4 4/15/2015 EXAMPLE • % mass = 2.50 grams NaOH x 100 (2.50 + 50.00) g solution • = 4.76% TRY • What is the percent by mass of a solution containing 12.3 g of caffeine dissolved in 100.0 g of water? • Step 1: Find the mass of the solution • 100.0 g + 12.3 g = 112.3 g TRY PERCENT VOLUME • Step 2: Divide the mass of the solute by the mass of the solution and multiply by 100 % • Sometimes we might not have solids, but liquids • Ratio of the volume of an ingredient by the total volume expressed as a percent • (12.3 / 112.3) x 100 • 11.0% • Ex. Rubbing Alcohol 70% Ethanol • This means 70% is Alcohol, 30% is water • % volume = volume of solute Volume of solution x100 5 4/15/2015 EXAMPLE • What is the percent by volume of alcohol if 50.0 ml of ethanol is diluted with water to form a total volume of 300.0 ml? 50.0 ml x100 300.0ml = 16.7 % alcohol TRY • If 19 mL of alcohol are dissolved in 31 mL of water, what is the percentage by volume of alcohol? • (19/50) = x 100 38% 6