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The Concentration of Substances VCE Chemistry Unit 2: Environmental Chemistry

The Concentration of Substances VCE Chemistry Unit 2: Environmental Chemistry Area of Study 1 – Water. Concentration. Concentration is usually defined in terms of the amount of solute per volume of solvent.

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The Concentration of Substances VCE Chemistry Unit 2: Environmental Chemistry

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  1. The Concentration of Substances VCE Chemistry Unit 2: Environmental Chemistry Area of Study 1 – Water

  2. Concentration • Concentration is usually defined in terms of the amount of solute per volume of solvent. • The greater the quantity of solute added to the solvent, the more concentrated the solution becomes. • When there is only a very small amount of solute present, the solution may be called dilute. • A concentrated solution may be diluted by the addition of more solvent.

  3. Concentration

  4. Expressing Concentration • There are many ways to express concentration units. • A common form is the ratio of the units of solute to the units of volume (e.g. g/L and mg/L). Conc. (C) = mass of solute (m) / volume of solution (V) • A fencepost preservative solution is prepared by dissolving 4.00 g of zinc chloride in enough water to make 2250 mL of solution. Find the concentration (C) of this solution in g/L and then in mg/L.

  5. Expressing Concentration V = 2250 mL = 2250 / 1000 = 2.25 L Concentration = mass / volume Concentration = 4.00 g / 2.25 L Concentration = 1.78 g/L zinc chloride Since there are 1000 mg in a 1 g Concentration = 1.78 x 1000 mg/L Concentration = 1780 mg/L zinc chloride

  6. Percentage by Mass (% w/w) • Percentage by mass describes the mass of solute, measured in grams, present in 100 grams of the solution. % w/w = mass of solute / mass of solution x 100% • Calculate the concentration (%w/w) for a solution of 38.4 g of NaCl in 150.0 g of solution % w/w NaCl = 38.4 g / 150 g x 100 % w/w NaCl = 25.6 %

  7. Percentage by Volume (% v/v) • The abbreviation v/v indicates that the percentage is based on volumes of both solute and solution. • The same units must be used to record both volumes. % v/v = volume of solute / volume of solution x 100% • A particular brand of champagne has an alcohol concentration of 11.5% v/v. What is the alcohol content of a 750 mL bottle? volume solute = % v/v . volume solution volume alcohol = (11.5 /100) x 750 mL volume alcohol = 86.25 mL

  8. Percentage Mass / Volume (% m/v) • Percentage mass/volume describes the mass of solute, measured in grams, present in 100 millilitres of the solution. % m/v = mass of solute / volume of solution x 100% • Find the % w/v of a saline solution consisting of 4.30 g of sodium chloride dissolved in 0.500 L of aqueous solution. V = 0.500 L = 500 mL % w/v saline = 4.30 g / 500 mL x 100% % w/v saline = 0.86 %

  9. Parts Per Million (ppm) • When very small quantities of solute are dissolved in a solution, the concentration can be measured in parts per million (ppm). • This unit can be written as mg/kg, mg/g, mg/L and mg/mL. • If 10 g of chlorine gas are dissolved in every 2 500 000 L of water, express the concentration of the chlorine water in ppm (μg/g). m (chlorine) = 10 g = 10 x 106mg = 1 x 107mg. m(water) 2,500,000 L ≈ 2 500 000 kg = 2 500 000 000 g Concentration = 1 x 107mg / 2.5 x 109 g = 4.0 x 10-3mg/g

  10. Unit Conversions

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