1 Concentration Calculations

1 Concentration Calculations - CONCENTRATION CALCULATIONS...

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CONCENTRATION CALCULATIONS 1. Calculate the % by mass of a solution that contains: (a) 5 mol HNO 3 in 750 g of solution (b) 5 mol HNO 3 in 750 g of H 2 O For both calculations we first need the mass of solute: mass of HNO 3 = 5 mol HNO 3 × (63.0 g HNO 3 / 1 mol HNO 3 ) = 315 g HNO 3 (a) % mass = (mass of solute/mass of solution) × 100 = (315 g / 750 g) × 100 = 42.0 % (b) % mass = (mass of solute/mass of solution) × 100 = (315 g / (750 + 315)g) × 100 = 29.6 % Note the subtle difference between the two parts of the question: the definition of %mass doesn’t change, so we need to be aware of always using the correct value for mass of solution. 2. A particular mouthwash contains 8 % alcohol by volume. Calculate the molarity of alcohol (C 2 H 5 OH) in this solution. (Assume the density of solute C 2 H 5 OH is 0.789 g/mL). 8% by volume: 8 mL C 2 H 5 OH in 100 mL of solution the 8 mL needs to be converted to a mass and then to number of moles…: 8mL C 2 H 5 OH = (8 mL C 2 H 5 OH) × (0.789 g C
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1 Concentration Calculations - CONCENTRATION CALCULATIONS...

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