AP Chemistry Friday Review #4 Directions: Complete the following problems on a separate sheet of paper. Show all relevant work. 1. Commercially available iodine tablets are frequently used for water purification by hikers and as a nutritional supplement. A chemist is tasked with determining the iodide concentration of a tablet that also contains a water-‐soluble sugar as a filler. A tablet is dissolved in 100.0 mL of distilled water and an excess of 0.10 M Pb(NO3)2 (aq) is added to the solution. The precipitate PbI2 forms, which is then filtered, washed, and dried. The data from the experiment are shown in the table below. Mass of KI tablet 0.500 g Mass of filter paper 1.362 g st Mass of filter paper + precipitate after 1 drying 1.770 g nd Mass of filter paper + precipitate after 2 drying 1.688 g Mass of filter paper + precipitate after 3rd drying 1.687 g a. Write the balanced, net-‐ionic equation for the reaction that occurs. b. In the filtrate solution, is the [K+] greater than, less than, or equal to [NO3-‐]? Justify your answer. c. Calculate the number of moles of precipitate that is produced in the experiment. d. Calculate the mass of I-‐ in the tablet. e. Calculate the mass percent of I-‐ in the tablet. f. In another trial, the chemist dissolves an identical tablet in 150.0 mL of distilled water instead of 100.0 mL of distilled water. Will the mass percent of I-‐ be greater than, less than or equal to the amount calculated in (d)? Briefly explain your answer.
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2. Calcium carbonate, CaCO3, is an abundant mineral on the Earth and is a major component in limestone, marble and seashells. Calcium carbonate has a limited solubility in water but its solubility is greatly enhanced when the water is acidic. The calcium carbonate in limestone gradually deteriorates in a hydrochloric acid solution according to the following equation: CaCO3(s) + 2HCl(aq) → CaCl2(aq) + CO2(g) + H2O(l) A student conducts an experiment to determine the mass percent of calcium carbonate in a limestone sample. The student places a 10.0-‐gram sample of limestone in a 250 mL Erlenmeyer flask fitted with a stopper and a plastic syringe as shown at right. This apparatus allows for the collection of the gaseous carbon dioxide product. The student adds 50.0 mL of a 6.0 M HCl(aq) solution to the limestone sample and quickly fits the flask with the gas collection apparatus. a. Write the net ionic equation for the reaction that occurs in the flask. b. Why is it important that the HCl(aq) be the excess reactant for this experiment?
c. Is the HCl the excess reactant? Justify your response with a calculation. (Hint: assume that the 10.0 g sample is made completely of calcium carbonate) d. The student monitors the syringe and observes that the maximum volume of carbon dioxide gas (d = 1.97 g/L) produced was 23.45 mL. i. Calculate the mass of calcium carbonate in the limestone sample. ii. Calculate the mass percentage of calcium carbonate in the limestone sample.
iii. Calculate [Cl-‐], the chloride concentration in the flask after the reaction is completed.
e. Will the following experimental error increase, decrease or have no affect on the mass percentage of calcium carbonate in the limestone sample? Briefly explain your response. i. The stopper was not quickly secured on the flask upon addition of the HCl(aq).
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