Name Date Directions: Read each question carefully and write your response in the space provided following each question. Your responses to these questions will be scored on the basis of the accuracy and relevance of the information cited. Explanations should be clear and well organized. Specific answers are preferable to broad, diffuse responses. For calculations, clearly show the method used and the steps involved in arriving at your answers. It is to your advantage to do this, since you may obtain partial credit if you do and you will receive little or no credit if you do not. Pay attention to units and significant figures. 1. A student is assigned the task of determining the mass percent of silver in an alloy of copper and silver by dissolving a sample of the alloy in excess nitric acid and then precipitating the silver as AgCl. First, the student prepares 50. ml of 6 M HNO 3. (a) The student is provided with a stock solution of 16 M HNO 3, two 100 ml graduated cylinders that can be read to ±1 ml, a 100 ml beaker than can be read to ±10 ml, safety goggles, rubber gloves, a glass stirring rod, a dropper, and distilled H 2 O. (i) Calculate the volume, in ml, of 16 M HNO 3 that the student should use for preparing 50. ml of 6 M HNO 3. (ii) Briefly list the steps of an appropriate and safe procedure for preparing the 50. ml of 6 M HNO 3. Only materials selected from those provided to the student (listed above) may be used. Page 1 of 9
(iii) Explain why it is not necessary to use a volumetric flask (calibrated to 50.00 ml ±0.05 ml) to perform the dilution. (iv) During the preparation of the solution, the student accidentally spills about 1 ml of 16 M HNO 3 on the bench top. The student finds three bottles containing liquids sitting near the spill: a bottle of distilled water, a bottle of 5% NaHCO 3 (aq), and a bottle of saturated NaCl(aq). Which of the liquids is best to use in cleaning up the spill? Justify your choice. Page 2 of 9
Then the student pours 25 ml of the 6 M HNO 3 into a beaker and adds a 0.6489 g sample of the alloy. After the sample completely reacts with the acid, some saturated NaCl(aq) is added to the beaker, resulting in the formation of an AgCl precipitate. Additional NaCl(aq) is added until no more precipitate is observed to form. the precipitate is filtered, washed, dried, and weighed to constant mass in a filter crucible. The data are shown in the table below. Mass of sample of copper-silver alloy Mass of dry filter crucible Mass of filter crucible and precipitate (first weighing) Mass of filter crucible and precipitate (second weighing) Mass of filter crucible and precipitate (third weighing) 0.6489 g 28.7210 g 29.3587 g 29.2599 g 29.2598 g (b) Calculate the number of moles of AgCl precipitate collected. (c) Calculate the mass percent of silver in the alloy of copper and silver. Page 3 of 9
2. For each of the following four reactions, write a balanced equation for the reaction in part (i) and answer the question about the reaction in part (ii). In part (i), coefficients should be in terms of lowest whole numbers and be sure to include states. Represent substances in solutions as ions if the substances are extensively ionized. Omit formulas for any ions or molecules that are unchanged by the reaction. You may use a separate sheet of paper for scratch work, but only equations that are written in the answer boxes provided will be scored. (a) Solid magnesium hydroxide is added to a solution of hydrobromic acid. (There are no solid products.) (i) Balanced equation: (ii) What volume, in ml, of 2.00 M hydrobromic acid is required to react completely with 0.10 mol of solid magnesium hydroxide? (b) A copper wire is dipped into a solution of silver nitrate. Assume all copper reacts as copper(ii). (i) Balanced equation: (ii) Describe what is observed as the reaction proceeds. Page 4 of 9
(c) Zinc metal is added to a perchloric acid solution. (There are no solid products.) (i) Balanced equation: (ii) Which substance is oxidized in the reaction? (d) A 20.0 ml sample of 0.10 M potassium phosphate is added to a 30.0 ml sample of 0.10 M calcium chloride. (i) Balanced equation: (ii) How many moles of product are formed? Page 5 of 9
3. An 8.55 mol sample of methanol, CH 3 OH, is placed in a 15.0 L evacuated rigid tank and heated to 327 C. At that temperature, all of the methanol is vaporized and some of the methanol decomposes to form carbon monoxide gas and hydrogen gas, as represented in the equation below. CH 3 OH(g) CO(g) + 2 H 2 (g) (a) The reaction mixture contains 6.30 mol of CO(g) at equilibrium at 327 C. (i) Calculate the number of moles of H 2 (g) in the tank. (ii) Calculate the number of grams of CH 3 OH(g) remaining in the tank. Page 6 of 9
(iii) Calculate the mole fraction of H 2 (g) in the tank. (iv) Calculate the total pressure, in atm, in the tank at 327 C. Page 7 of 9
(b) Consider the three gases in the tank at 327 C: CH 3 OH(g), CO(g), and H 2 (g). (i) How do the average kinetic energies of the molecules of the gases compare? Explain. (ii) Which gas has the highest average molecular speed? Explain. Page 8 of 9
(c) The tank is cooled to 25 C, which is well below the boiling point of methanol. It is found that small amounts of H 2 (g) and CO(g) have dissolved in the liquid CH 3 OH. Which of the two gases would you expect to be more soluble in methanol at 25 C? Justify your answer. Page 9 of 9