Chapters 1-4 Example Problems

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Preston Mosley
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1 Chapters 1-4 Example Problems 1. Lead ions (particularly Pb 2+ ) pose a neurological hazard, particularly to babies and toddlers, and can be found in high concentrations in the environment due to lead paint, leaded gasoline, and old plumbing. In my Analytical Chemistry class, we analyze urban soil samples for lead(ii) using an atomic absorption spectrometer. Calibrating this spectrometer requires the preparation and use of aqueous solutions of lead(ii) nitrate. When we are done with the Pb(NO 3 ) 2 (aq) solutions, we wish to precipitate the Pb 2+ so that we can dispose of it safely. (a) Draw a cartoon of an aqueous solution of lead(ii) nitrate. (b) Identify an anion that will precipitate the Pb 2+. (c) Identify a salt which is soluble in water and that contains the anion identified in part (b). (d) Write a balanced molecular equation to describe the chemical reaction of Pb(NO 3 ) 2 (aq) and the compound you identified in part (c). Write the phases of all species. (e) Write a balanced complete ionic equation for the same chemical reaction. Write the phases of all species. (f) Write a balanced net ionic equation for the same chemical reaction. Write the phases of all species Page 1 of 8

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3 2. The perchlorate ion, a major component of solid rocket fuel (and a minor constituent of air exposed to lightning), may inhibit thyroid functioning. The Environmental Protection Agency (EPA) has recently articulated a safe maximum exposure for ClO - 4 : A person should ingest no more than 0.7 µg ClO - 4 per kg of body weight per day (Chemical and Engineering News, 28 February 2005, p. 14). (a) At 10 months old, my child Margaret weighed 16 lb. According to the EPA standard, what is the maximum weight of ClO - 4 ions (in µg) that a 16-lb person should ingest per day? (Note that 1 kg = lb. Also note that there must, of course, be enough cations around to balance the charge of the ClO - 4 anions. However, the chemical identities of these cations are irrelevant to these calculations.) (b) The EPA supervises the clean-up of especially polluted sections of land known as Superfund sites. At these sites, the concentration of ClO - 4 must be reduced to a mixing ratio no higher than 18 parts per billion (ppb), 9 y g ClO 4 y x 10 g ClO4 where y ppb means = 10 9 g H 2 O g H 2 O Based on your answer to part (a), how many fluid ounces (fl oz) of water could Margaret safely drink per day if the water contained 18 ppb of ClO - 4? (Note: the density of H 2 O(l) is 1.0 g H 2 O/mL H 2 O, and 1 ml = fl oz.) (c) A common way to report the concentration of a solute (X) in a solution is as a molarity (M). This is defined as mol X [X] = L solution Calculate the molarity of an 18 ppb solution of ClO - 4 assuming that the density of the solution is the same as the density of pure H 2 O(l). (d) How many ClO - 4 ions will there be in an 8 fl oz Sippy Cup containing water with 18 ppb ClO - 4? Page 3 of 8

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5 3. What good is the perchlorate ion? The following balanced chemical reaction describes what happens inside the two solid rocket boosters in the Space Shuttle upon launch: 10 Al(s) + 6 NH 4 ClO 4 (s) 4 Al 2 O 3 (s) + 2 AlCl 3 (s) + 12 H 2 O(g) + 3 N 2 (g) This extremely exothermic (i.e. heat-releasing) reaction causes the boosters to reach a temperature of 3500 K. The water vapor and N 2 therefore exert a tremendous amount of upward pressure (PV = nrt; Chapter 5) enough to lift the shuttle to an altitude of 28 miles. (a) How many total moles of gas can form when 300. g of Al and 300. g of NH 4 ClO 4 react? (b) When the reaction has gone to completion, how many grams of Al and NH 4 ClO 4 remain? (c) Let s consider together what makes this reaction (i) chemically favorable and (ii) effective at propulsion. Page 5 of 8

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7 4. The most basic description of a substance s chemical composition is its empirical formula, which indicates the relative numbers of each kind of element present in a molecule (or formula unit, for an ionic compound) of that substance. By convention, we write an empirical formula using the smallest set of whole-number subscripts. The molecular formula of a compound, which indicates the absolute numbers of each kind of element present in a molecule of a substance, may be different, as this problem shows. The empirical formula of an organic substance is often determined by combustion, or reaction with oxygen gas. Say an unknown substance containing C, H, and O combusts according to the following chemical equation: unknown(s) + O 2 (g) CO 2 (g) + H 2 O(g) In one experiment, g of unknown underwent complete combustion to form g CO 2 and g H 2 O. No other products were formed. (a) Determine the mass percent of oxygen in the unknown. (b) Find the empirical formula of the unknown. (c) In a separate experiment, the molar mass of the unknown is determined to be 152 g mol -1. Find the molecular formula of the unknown. (d) Balance the combustion equation. (e) Now that we know the composition of this substance, what do we know about its structure? Page 7 of 8

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Name AP Chemistry September 30, 2013

Name AP Chemistry September 30, 2013 AP Chemistry Exam Part I: 40 Questions, 40 minutes, Multiple Choice, No Calculator Allowed Bubble the correct answer on the blue side of your scantron for each of the