8.1. Concentration of solutions The concentration of a solution indicates the amount of solute present in a certain amount of solution or solvent. The concentration of a solution can be expressed in dif ferent ways. 28 We prepare two solutions: one by dissolving 10 g of sugar in 100 g of water, and the other by dissolving 5 g of sugar in 20 g of water. Which solution is more concentrated? Which one would taste sweeter? 29 To prepare a physiological saline solution, we dissolve 9 g of salt in 1 litre of water. a) Calculate the concentration expressed as a percentage by mass. b) How much salt would you need to prepare 200 g of physiological saline solution? 30 To prepare a disinfectant, we mix 200 mL of distilled water with 400 mL of ethyl alcohol and 10 mL of benzyl alcohol. Calculate the concentration of each type of alcohol, expressing it as a percentage by volume. Assume that the volumes are additive. 31 According to the WHO, for a hydroalcoholic gel to have a virucidal effect (it destroys viruses) it must contain between 70 % and 90 % alcohol. What would you have to add to the disinfectant in the previous activity for it to have a virucidal effect? How much of it? A C T I V I T I E S 1 Percentage (%) by mass (or richness) It indicates the mass of solute per 100 units of mass of solution . they must be expressed in the same units % of solute by mass = ⋅ mass of solute mass of soluti n 100 o Percentage (%) by volume It indicates the volume of solute per 100 units of volume of solution . they must be expressed in the same units % of solute by volume = ⋅ volume of solute volume of solution 100 SOLVED PROBLEM 7 We prepare a solution by mixing 100 g of water, 10 g of sugar and 5 g of salt. a) What is the percentage of sugar by mass? b) What is the percentage of salt by mass? a) Percentage of sugar by mass: % msugar = + + ⋅ = 10 g of sugar (100 10 5 g) of solution 100 8.7 % b) Percentage of salt by mass: % msalt = + + ⋅ = 5 g of salt (100 10 5 g) of soluti n 100 o 4.35 % SOLVED PROBLEM 8 A wine is 14 % alcohol by volume. What volume of alcohol does a person consume when they drink a 150 mL glass of wine? % Valcohol = ⋅ V V alcohol solution 100 Substitute the data: 14 100 14 100 = ⋅ = = ⋅ V V alcohol alcohol 150 mL 150 mL ; 21 mL The problem can also be solved with conversion factors: 150 mL of wine 14 mL of alcohol 10 mL of wine ⋅ =21 mL of alcohol 25
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