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Understanding Concentration: Percentages, ppm, ppb, and Molarity Explained

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12 diapositives · 398 Views · Oct 11, 2014

This guide explores the concept of concentration in solutions, detailing methods to express it including percentages (V/V, W/W, W/V), parts per million (ppm), and parts per billion (ppb). Readers will learn the calculations needed to determine the percentage concentrations using various examples, such as acetic acid in a solution and the composition of alloys like brass. Additionally, it covers molar concentration, a crucial term in chemistry, allowing for conversion of moles to volume in liters. This comprehensive overview is essential for mastering solution concentration.

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Understanding Concentration: Percentages, ppm, ppb, and Molarity Explained
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  1. Concentration

  2. Solution Concentration Concentration = quantity of solute quantity of solution (not solvent) There are 3 basic ways to express concentration: 1) percentages, 2) ppm and ppb (used for very low concentrations) 3) molar concentrations

  3. 1. Percent concentration • % concentration can be in • 1a) % V/V • 1b) % W/W • 1c) % W/V

  4. 1a) Percent volume by volume concentration %V/V = volume of solute (mL or L) X 100% Volume of solution (mL or L) 1.140 mL of acetic acid is present in a 500 mL bottle of solution? What is the percentage by volume concentration of acetic acid? 28% V/V 2. Gasohol fuel is a solution of ethanol and gasoline. A typical gasohol mixture available across Canada contains 4.1 L of ethanol in a 55-L tank of fuel. What is the percentage by volume concentration of ethanol? 7.5 3. What is approximate % V/V if 30 mL of pure ethanol is added to 250 mL of water? 11% V/V

  5. 1b) Percent weight by weight concentration • %W/W = Weight of solute (g or kg) X100% • Weight of solution (g or kg) • What is the % W/W if 8.0 g copper is added to enough zinc to produce 100 g of an alloy? 8% W/W. • Brass is a copper-zinc alloy. If the concentration of zinc is relatively low, the brass has a golden color and is often used for inexpensive jewellery. If a 35.0 g pendant contains 1.7 g of zinc, what is the percentage weight by weight of zinc in brass?4.9% W/W • What is the % W/W of copper in an alloy when 10 kg of Cu is mixed with 250 kg of Zn? 3.8 % W/W

  6. 1c) Percent weight by volume concentration • %W/V has different units – weight and volume. What does 3% H2O2 W/V mean? • % W/V = weight of solute (g) X 100% volume of solution (mL) • Can Use g/ml or kg/L • Solder flux, available at hardwood stores, contains 16 g of zinc chloride in 50 ml of solution. The solvent is HCl (aq.). What is the percentage weight by volume of zinc chloride in the solution? 16 g / 50 mL = 32% W/V • If 250 g of sodium chloride is present in 3 L of solution, what is the percentage weight by volume of sodium chloride? 250g/3000ml = 8.5% W/V

  7. 2. Very low concentrations • Usually expressed in parts per million (ppm) or parts per billion (ppb) • Is a special case of the W/W ratio Ex. chlorine in a swimming pool may be expressed as 1 ppm OR 1 g of Cl in 106 g pool water ((1g/ml)) 1 mg/L1 ppm 1 mg/kg

  8. 2. Very low concentrations • Parts per billion is 1 part in _______ parts? • 1000 times smaller than ppm. • Use mg/1000L • mg/1000 kg • Do questions 5-7 on pg. 284

  9. 3. Molar concentration Molar concentration is the most commonly used in chemistry. Molar concentration = amount of solute (in moles) (M) Volume of solution (in L) M = n/V or M = n/L Ensure that units are mol/L. DO question #8 on page 284

  10. Molar concentration Q) If a reaction produced 0.186 mol of sodium hydroxide in 0.250 L of solution. Calculate the molar concentration of sodium hydroxide. 0.186 mol/0.250 L = 0.744 mol/l or 0.744 M

  11. Percentage concentration (V/V, W/V, W/W), very low concentration, molar concentration • % V/V = 4.1 L / 55 L = 7.5% V/V • % W/V = 16 g / 50 mL = 32% W/V • % W/W = 1.7 g / 35.0 g = 4.9% W/W • 8 ppm = 8 _ / 1 L, the units should be 1 million times smaller than 1L (or 1kg): 8ppm = 8 mg/L so the mass in 1 L is 8 mg. • 3.2 mg/0.59 kg = 5.424 mg/kg = 5.4 ppm • a) ppb is 1000 times smaller than ppm b) 1 g/109 mL, 1 mg/1000 L, 1 g/L, 1 mg/kL, 1 mg/Mg, 1 g/kg • 0.11 mol / 0.060 L = 1.8 mol/L

  12. Text book pg. 287 #11-17 Answers • 500 mL x 70 mL/100 mL = 350 mL • 250 mL x 1000 = 250 L 250 L x 3 kg/100 L = 7.5 kg • 1.5 ppm = 1.5 mg/L 0.250 L x 1.5 mg/L = 0.38 mg • 75 L x 0.055 mol/L = 4.1 mol • 0.050 L x 5.0 mol/L = 0.25 mol • 0.500 mol x 1 L/1.24 mol = 0.403L or 403mL • 0.14 mol x 1 L/2.6 mol = 0.0538 L = 54 mL