1. Calculate the equilibrium pH of the following solutions: a.0.250 M H 2 CrO 4
ID: 534707 • Letter: 1
Question
1. Calculate the equilibrium pH of the following solutions:
a.0.250 M H2CrO4
b. 0.250 M NaHS
c. Write charge balance and mass balance equations for the solution in part b.
d. Design a buffer that maintains a pH of 4.75. What components would you use, and what would thier concentrations be?
2. a. Food rots about 40 times faster at room temperature (25 degrees Celsius) than when it is refrigerated at 4 degrees Celsius. Estimate the overall activation energy for the prcoesses involved in its decomposition.
b. Why does gasoline not spontaneouosly combust inside the gas tank of the car? (i.e. Why does a thermodynamically favorable reaction not always happen spontaneoulsy under standard conditions?)
Explanation / Answer
1. pH
a. 0.250 M H2CrO4
H2CrO4 <==> H+ + HCrO4-
Ka1 = [H+][HCrO4-]/[H2CrO4]
0.18 = x^2/(0.250 - x)
x^2 + 0.18x - 0.045 = 0
x = [H+] = 0.14 M
pH = -log[H+] = 0.85
b. 0.250 M NaHS
NaHS --> Na+ + HS-
HS- + H2O <==> H2S + OH-
Kb2 = 1 x 10^-14/9.5 x 10^-8 = x^2/0.250
x = [OH-] = 1.62 x 10^-4 M
pOH = -log[OH-] = 3.79
pH = 14 - pOH = 10.21
c. charge balance [HS-] = [H+]
mass balance 0.250 M [HS-] = [H2S] + [OH-]
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