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Unformatted text preview: spano velasco (ds28282) hw 02 Fakhreddine (52395) 1 This print-out should have 36 questions. Multiple-choice questions may continue on the next column or page find all choices before answering. 001 1.0 points We have an aqueous solution that contains 26% (by mass) of a hypothetical solute Z. The formula weight of the solute Z is 175 g/mol. The density of the solution is observed to be 1 . 4 g/mL. What is the molarity of Z in this solution? Correct answer: 2 . 08 mol / L. Explanation: % of Z = 26% FW Z = 175 g / mol density soln = 1 . 4 g / mL The solution is 26% Z by mass. We can write this as the ratio 26 g Z 100 g solution . The formula weight can be written as 175 g Z 1 mol Z . The density of the solution is 1 . 4 g/mL, more fully written as 1 . 4 g soln 1 mL soln . We want to calculate the molarity of Z, which has the units of moles Z per liter so- lution. The density value already has the volume of solution in the denominator, so we start with the value and convert from mL to L: ? g soln 1 L soln = 1 . 4 g soln 1 mL soln 1000 mL soln 1 L soln = 1400 g soln 1 L soln We use the percent by mass to convert the numerator from grams solution to grams Z, then we use the formula weight of Z to convert from grams to moles Z: ? mol Z 1 L soln = 1400 g soln 1 L soln 26 g Z 100 g soln 1 mol Z 175 g Z = 2 . 08 mol / L 002 1.0 points What additional information, if any, would enable you to calculate the molality of a 7.35 molar solution of a nonelectrolyte solid dis- solved in water? 1. Only the density of water would be needed. 2. Only the molecular weight of the solute would be needed. 3. None is needed. 4. Only the density of the solution would be needed. 5. Both the density of the solution and the molecular weight of the solute would be needed. correct Explanation: molarity = mol solute L solution molality = mol solute kg solvent The density of the solution can be used to convert volume (1 L) of solution into mass of solution. Then the molecular weight of the solute (given or calculated from the formula) can be used to convert the number of moles solute in 1 L solution into mass of solute in grams. The mass of the solvent is the difference between the mass of the solution and the mass of the solute (both of which have been calculated). Substitute the values into the molality formula and calculate. 003 1.0 points The mole fraction of a certain nonelectrolyte compound in a solution containing only that substance and water is 0.100. The molecular weight of water is 18.0 g/mol. What addi- tional information is needed to determine the molality of the solution? 1. No additional information; the molality can be calculated from the information given....
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This note was uploaded on 04/08/2009 for the course CH 302 taught by Professor Holcombe during the Spring '07 term at University of Texas at Austin.
- Spring '07