AP Chemistry Unit 2 Test (Chapters 3 and 4)

Transcription

1 AP Chemistry Unit 2 Test (Chapters 3 and 4) NAME: 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 16 M HNO3 nitric acid in a beaker and then precipitating the silver as AgCl with a M NaCl solution prepared by the student. a. During the dissolving of the alloy, the student accidentally spills about 2-3 ml of 16 M HNO 3 on the bench top. The student finds three bottles containing liquids sitting near the spill: a bottle of sodium hydroxide, a bottle of sodium hydrogen carbonate and a bottle of sodium chloride. Which of the solids is best to use in cleaning up the spill? Justify your choice. Include a balanced equation for the reaction that takes place cleaning up the spill. b. During the dissolving of the alloy describe any signs that a chemical reaction has taken place that indicate that the alloy contained copper. c. Using the ½ reaction method write a balanced chemical equation for the reaction between the copper and the nitric acid. d. The student then needs to prepare 250 ml of a M NaCl solution. The student is provided with a stock bottle of solid sodium chloride, a 250 ml graduated cylinder that can be read to ±2 ml, a 250 ml beaker that can be read to ±10 ml, a 250 ml volumetric flask that can be read to ±0.16 ml, an analytical balance that can be read ± g, an electronic balance that can be read ±0.1 g safety goggles, a weighing dish, a wash bottle, and distilled H 2 O. Calculate the mass of sodium chloride needed and briefly list the steps of an appropriate and safe procedure for preparing the 250 ml of a M NaCl solution.

2 Initial Volume in buret Volume in buret after first settling Volume in buret after second settling Volume in buret after third settling (no more precipitate forms) 2.46 ml ml ml 24.50mL e. Then the student adds 16 M HNO 3 into a beaker that has g sample of the alloy until all the alloy dissolves. After the sample completely reacts with the acid, the alloy solution is transferred into an Erlenmeyer flask and a titration is performed. The prepared sodium chloride solution is added using a buret until no more precipitate forms after allowing the precipitate to settle to the bottom of the flask. The data for that titration is in the table above. i. Write a complete balanced equation for the reaction that takes place in the Erlenmeyer flask. ii. Calculate the mass of the AgCl precipitate made. iii. Calculate the mass percent of silver in the alloy of copper and silver. iv. Explain why your percentage may or may not be exact.

3 2. Iron metal can be made from iron(iii) oxide by reacting the iron(iii) oxide with carbon monoxide with the other product of this reaction being carbon dioxide. A 16.2 L sample of carbon monoxide at 1.50 atm and 200. o C is combined with g of iron(iii) oxide. a. Write a balanced chemical equation for the reaction. Circle the oxidizing agent and label it OA. b. Calculate the moles of each reactant. i. Carbon monoxide ii. Iron(III) oxide c. What is the limiting reactant for the reaction? Justify your answer with calculations. d. How many grams of iron will be made? A water solution of iron is found in the lab, but what is not know about the solution is what kind of iron ions it contains, mainly iron (II) ions or iron (III) ions. e. What visual observations could help in identifying the ion type? f. What substance or substances could be added to confirm this? Write a brief procedure of what you could add and what visual observations you could use in your determination.

4 3. Answer the following questions that relate to the analysis of chemical compounds. a. A compound containing the elements C, H, N, O is analyzed. When a g sample is burned in excess oxygen gas, g of CO 2(g) is formed. The combustion analysis also showed the sample contained g of H. i. Determine the mass, in grams, of C in the g sample of the compound. ii. When the compound is analyzed for N content only, the mass percent of N is found to be percent. Determine the mass, in grams, of N in the original g sample of the compound. iii. Determine the mass, in grams, of O in the original g sample of the compound. iv. Determine the empirical formula of the compound. b. A different compound, which has the empirical formula CH 2 Br, has a vapor density of 6.00 g L-1 at 375 K and atm. Using these data, determine the following. i. The molecular mass of the compound with appropriate units ii. The molecular formula of the compound

5 4. Equation Section: Write the formulas to show the reactants and products for ALL of the laboratory situations described below. In all cases a reaction occurs. Assume that solutions are aqueous unless otherwise indicated. Represent substances in solution as ions if the substances are extensively ionized. Omit formulas for any ions or molecules that are unchanged by the reaction. You need to balance the equations, and then answer the follow-up question about the reaction. Example: A strip of magnesium is added to a solution of silver nitrate. Mg + Ag + Mg 2+ + Ag a. Bromine water is mixed through a solution of potassium iodide. ii. Who is the oxidizing agent? b. Solid magnesium carbonate is added to a test tube containing acetic acid. ii. How could you determine when the all the acid has reacted? c. A solution of silver nitrate is added to a solution of sodium chloride. a. What would you see when the reaction takes place inside a test tube? d. Carbon dioxide gas is bubbled through water. ii. What happens to the ph of the water?

6 e. Solid copper hydroxide is heated in test tube. ii. What color changes would be observed during the reaction? f. A solution of chromium (III) chloride and then hydrogen peroxide are mixed in a basic solution. ii. What color changes would be observed during this reaction? g. Chlorine water and a solution of sodium sulfite are mixed in an acidified solution. iii. What cation could be added to confirm the presence of the anions containing sulfur that is made during the reaction?

7 5. Discuss the following phenomena in terms of the chemical and physical properties of the substances involved and general principles of chemical and physical change. a. What will be observed on the surfaces of zinc and silver strips shortly after they are placed in separate solutions of CuSO 4, as shown in the picture below. Account for these observations. b. Chlorine water, Cl 2, in a basic solution and hydrogen peroxide, H 2 O 2, in an acidic solution both undergo a reaction that is called a disproportionate reaction. Using the products of each of these two separate reactions to explain the definition of a disproportionate reaction. 6. Given solid samples of NaI and of AgNO 3, briefly describe two simple laboratory tests by which these two compounds can be distinguished. For each test, report the expected result for each compound.