55 The label on a bottle of perfume indicates that it is 80 % alcohol by volume. Calculate the amount of alcohol needed to prepare the 280 mL of perfume in the bottle. 56 Some "non-alcoholic" beers can contain up to 1 % alcohol. In a half-litre bottle of this type of beer, how much alcohol will there be? 57 C H E M I S T R Y A N D C O O K I N G Spherification is a commonly seen molecular cooking technique. It was popularised by chef Ferrán Adrià, although it has been known since 1946. It consists of enclosing a liquid inside a semi-solid membrane. To achieve this, a substance called sodium alginate must be added to the food. Drops of this mixture are then added to a bath of calcium chloride dissolved in water. After a few seconds, balls that resemble caviar can be collected. One of the keys to success is to be precise with the concentrations. You have to add 10 g of calcium chloride per litre of water. a) What is the molar concentration of the calcium chloride solution? b) How much calcium chloride do you need to prepare 100 cm3 of solution? S O LV E D P R O B L E M 1 3 Sodium sulphide (Na2S) is a water-soluble substance used as a bleach in the textile industry. a) H ow much Na2S do you need to prepare 500 mL of 1.5 M solution? b) W hat is the concentration of the sodium ion (Na+) in that solution? a) Calculate the amount of solute you need to prepare that amount of solution. M n V L s = → ( ) substitute the data: ns = M × V (L) = 1.5 M × 0.5 L = 0.75 mol To know the equivalent mass to these moles of solute, you must calculate their molar mass (M). M (Na2S) = 23 × 2 + 32.06 = 78.06 g/mol 0 75 1 . mol mol 78.06 g of Na S of Na S of Na S 2 2 2 ⋅ =58.55 g of Na S 2 b) When dissolved in water, Na2S separates into ions: Na2S ® 2 Na + + S2– Each mol of Na2S gives 2 mol of Na +. Therefore: The concentration of Na+ in the solution is: 2 × 1.5 M = 3 M 58 We prepare a solution by dissolving 15 mL of 1.5 M Na2S in water until it reaches 100 mL. What is the concentration of the new solution? 59 The solubility of some substances in water varies greatly with temperature. The graph shows the maximum amount of potassium nitrate (KNO3) that can be dissolved in 100 g of water at different temperatures. Solubility (g of solute/100 g of water) 20 30 50 60 70 80 90 40 100 200 180 160 140 120 100 80 60 40 20 0 T (°C) a) Read on the graph the solubility of KNO3 in water at 75 ºC. Express it as a percentage by mass. b) Determine whether we can prepare a solution of 30 % KNO3 in water at 45 ºC. c h e c k yo u r p r o g r e s s 32
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