Chemistry 251 Review Questions Exam I Word

Chemistry 251. Review questions. Exam I Questions 1. Seawater contains an average of 1.08x103 ppm of Na + and 270 ppm of SO 42-. Calculate the molar concentration of Na+ and SO42-. Given that the average density of seawater is 1.02 g/mL. 2. Write the answer with the correct number of digits? Find the absolute uncertainty and percent relative uncertainty for the following: [91.3 (±1.0) x 40.3 (± 0.2)] / 21.2 (±0.2) = ? 3. 0.5073 g of impure K 2S2O8 (molar mass 270.32 g/mol) was analyzed by treatment with excess standard Na2C2O4 by the following reaction:

K2S2O8 + Na2C2O4

→

Na2SO4 + K2SO4 + 2CO2

 After reaction with 50.00 mL of 0.05006 0.05006 M Na2C2O4, the excess oxalate required 16.52 mL of 0.02013 M KMnO 4 per the following reaction: 5HOOCCOOH + 2MnO4 + 6H+ → 10CO2 + 2Mn2+ + 8 H2O Find the weight percent (mass percent) of K 2S2O8 in the impure sample. 4. How many mL of 0.500 M NaOH should be added to 10.0 g of tris hydrochloride [(BH+Cl-), molar mass 157.60 g/mol, pKa = 8.07)] to give a pH of 7.60 in a volume of 250 mL. 5. The dibasic compound B (pKb 1 = 4.00), pKb2 = 8.00) was titrated with 1.00 M HCl. The initial solution of B was 0.100 M and had a volume of 100.0 mL. a) At what volumes are the two equivalence points? b) Find the pH at the following volumes of acid added V a : 5.0 mL, 15.0 mL. 6. A 7.25 x 10 -5 M solution of potassium permanganate has a transmittance of 44.1% when measured in a 2.10 cm cell at a wavelength of 525 nm. Calculate: a) the absorbance of this solution b) the molar absorptivity of KMnO4

7. Fill in the blanks: In the experiment, Colorimetric/spectrophotometric determination of phosphate in seawater, all glassware was washed with ________________ because  ___________________________. 8. Fill in the blanks: In the experiment, Turbidimetric determination of sulfate in seawater, the absorbance measurements were made at a wavelength of 420 nm. The frequency of electromagnetic radiation with a wavelength of 420 nm can be calculated from the  ____________________ equation. The frequency is _____________________ .

9. Fill in the blanks: In the experiment, Alkalinity of seawater, the titrations are done using H2SO4. The process of determining precisely the concentration of ~ 0.01 M H 2SO4 is called  _____________________. 10. A scientist wishes to prepare a 1 L sample with the approximate composition of seawater for use in the laboratory: What mass of NaCl, MgSO 4, MgCl2 and CaCl2 one +

-

would you use to match the concentrations of Na , Cl , Mg2+, Ca2+, and SO42- given

below? (density of seawater is 1.00g/mL) -

Cl , +

Na

18.980 ppt 10.560 ppt

SO42- 2.560 ppt Mg2+ 1.272 ppt Ca2+ 0.400 ppt 11. Salinity is defined by oceanographers as follows: (Mass of dissolved salts/mass of solution) x 1000, suggest a simple way to determine the salinity of seawater sample in the laborator y. 12. Write a NIE for titration of HCO 3- with H2SO4 to the second equivalence point. 13. A concentration of 8.9 µg of lead per 355 g of solution is equivalent to: 0.0000025 ppb 25 ppb 250 ppb 2500 ppb 0.0025 ppb 14. What is the emergency phone number in the lab?______________ 15. What mass of NaOH is present in 2.30 L of 5.40 molal NaOH (aq). The density of the solution is 1.15 g/mL.