CHEMpossible. 102 Exam 2 Review

Transcription

1 CHEMpossible 1. Which of the following statements is/are true for a 0.10 M solution of a weak acid HA? a. The ph is b. [H + ] > [A - ] c. [H + ] = [A - ] d. The ph is less than Consider the following acids: a. HClO 2 (pk a = 2.00) b. HCN (pk a = 9.31) c. HF (pk a = 3.45) d. HIO (pk a = 10.64) e. HClO (pk a = 7.53) Which acid will have the greatest percentage of solute ionized in a 0.10 M aqueous solution? 3. When g of an organic base of molar mass 162 g/mol is dissolved in 50.0 ml of water, the ph is found to be Calculate the pk b of the base and the pk a of the conjugate acid. 4. Sulfur hexafluoride is a colorless, odorless, very unreactive gas. Calculate the pressure (in atm) exerted by 1.82 moles of the gas in a steel vessel of volume 5.43 L at 69.5 C. 5. It takes 192 s for an unknown gas to effuse through a porous wall and 84 s for the same number of molecules of N 2 gas to effuse. What is the molar mass of the unknown gas? 6. Which one of the following salts gives an acidic solution? CaCl 2 NH 4Cl LiF CsNO 3 NaCH 3CO 2

2 Relative # of molecules A mixture of helium and methane gases is placed in an effusion apparatus. Calculate the ratio of their effusion rates. 8. The ph of coffee is approximately 5.0. How many times greater is the [H + ] in coffee than in neutral water (both at 25 C). a. 200 b. 100 c. 5.0 d. 1.4 e The graph below shows the distribution of molecular speeds for argon and helium at the same temperature. 1 2 Molecular speed f. Does curve 1 or 2 better represent the behavior of argon? g. Which curve represents the gas that effuses more slowly? h. Which curve more closely represents the behavior of fluorine gas? Explain. 10. Given the following data for weak acids. THe ph will be the highest for which of the salt solutions below? HF (pk a = 3.45) HClO 2 (pk a = 2.00) HNO 2 (pk a = 3.37) a M NaClO 2 b M NaNO 2 c M NaF

3 11. An inflated helium balloon with a volume of 0.55 L at sea level (1.0 atm) is allowed to rise to a height of 6.5 km, where the pressure is about 0.40 atm. Assuming that the temperature remains constant, what is the final volume of the balloon? 12. You have 500 ml of a buffer solution containing 0.20 M acetic acid (CH 3COOH) and 0.30 M sodium acetate (CH 3COONa). What will the ph of this solution be after the addition of 20.0 ml of 0.36 M NaOH solution? [K a = ] 13. Three 5-L flasks, fixed with pressure gauges and small valves, each contain 4 g of gas at 273 K. Flask A contains H 2, flask B contains He, and flask C contains CH 4. Rank the following contents in terms of a) pressure, b) average molecular kinetic energy, c) diffusion rate after the valve is opened, d) total kinetic energy of the molecules, and e) density. 14. Twenty-five ml of a 0.25 M solution of the weak acid benzoic acid (C 6H 5COOH, K a = ) is titrated with 0.40 M potassium hydroxide. Calculate the ph in each of the following additions of acid. a. 0 ml of titrant has been added

4 b. After 6 ml of base has been added c. At the equivalence point d. After 35 ml of base has been added e. Draw the titration curve. Be sure to label the axes, the buffer region, the pka, the equivalence point, a ph range for an indicator, and inflection point. f. Draw the Erlenmeyer and burette used in this titration. Label which species goes in each piece of glassware.

5 15. Consider the following gaseous sample in a cylinder fitted with a movable piston. Initially, there are n moles of the gas at temperature T, pressure P, and volume V. Choose the cylinder that correctly represents the gas after each of the following changes. a. The pressure on the piston is tripled at constant n and T. b. The temperature is doubled at constant n and P. c. N moles of another gas are added at constant T and P. d. T is halved and pressure on the piston is reduced to a quarter of its value. 16. Identify the Lewis acid and Lewis base in each of the follow reactions: a. C 2H 5OC 2H 5 + AlCl 3 (C 2H 5) 2OAlCl 3 b. Hg CN - 2- Hg(CN) 4 c. Co NH 3 Co(NH 3) A certain amount of gas at 25 C and at a pressure of atm is contained in a glass vessel. Suppose that the vessel can withstand a pressure of 2.00 atm. How high can you raise the temperature of the gas without bursting the vessel?

6 18. Calculate the ph of a 0.14 HNO 2 solution that is 5.7% ionized. a b c d e Dry ice is solid carbon dioxide. A g sample of dry ice is placed in an evacuated 4.6-L vessel at 30 C. Calculate the pressure inside the vessel after all the dry ice has been converted to carbon dioxide gas. 20. The percent by mass of bicarbonate (HCO 3- ) in a certain Alka-Seltzer tablet is 32.5 percent. Calculate the volume of CO 2 generated (in ml) at 37 C and 1.00 atm when a person ingests a 3.29-g tablet. (The reaction is between HCO 3 - and HCl acid in the stomach) 21. The volume of a sample of pure HCl gas was 189 ml at 25 C and 108 mmhg. It was completely dissolved in about 60 ml of water and titrated with an NaOH solution; 15.7 ml of the NaOH solution were required to neutralized the HCl. Calculate the molarity of the NaOH solution. 22. How does the kinetic-molecular theory of gases explain why 1 mol of krypton and 1 mol of helium have the same volume at STP?