For the process of a certain liquid vaporizing at 1 atm, delta H degree vap = 54
ID: 936197 • Letter: F
Question
For the process of a certain liquid vaporizing at 1 atm, delta H degree vap = 54.2 kJ/mol and delta S degree vap = 74.1 J/mol K. Assuming these values are independent of T, what is the normal point of this liquid? 0.731 degree C 458degree C 1004 degree C 731 degree C none of these Given 100.0 mL of a buffer that is 0.50 M in HOCl and 0.77 M in NaOCl, what is the pH 10.0 mL of 1.0 M NaOH has been added? (Ka for HOCl = 3.5 x 10-8) 7.68 7.79 7.46 7.12 7.74 :Assume an indicator works best when the equivalence point of a titration comes in the middle the indicator range. Which indicator would be best for the following titration? 0.100 M HOCl (Ka = 3.5 x 10-8) + 0.100 W NaOH crystal violet (0.2 -1.8) cresol red (7.0 - 8.8) methyl orange (3.2 - 4.4) thymolphthalein (9.5 - 10.5) phenolphthalien (8.2 - 10.0)Explanation / Answer
1) T boil = Delta H vap / Delta S vap = 54200 / 74.1 = 731.44 K = 731.44 - 273 = 458.4 degree C = B)
2) pH = pKa + log (Salt + X / Acid - X)
X = Amount of NaOH added
Salt = 0.77 x 0.1 = 0.077 moles
Acid = 0.5 x 0.1 = 0.05 moles
X = NaOH = 1 x 0.01 = 0.01 moles
pKa = - log Ka = 7.46
pH = 7.46 + log (0.077 + 0.01 / 0.05 - 0.01) = 7.79 = B)
3) For combination of Weak Acid and Strong Base,
pH = 0.5 x (pKw + pKa + log C)
pKw = 14
pKa = - log Ka = 7.46
Assuming equal volumes of HOCl and NaOH mixed,
C (after mixing) = 0.1 / 2 = 0.05
=> pH = 0.5 x (14 + 7.46 - 1.3) = 10.08
Since it lies between 9.5 and 10.5, best indicator is D) thymolphthalein