Consider a buffer solution consisting of CH 3 NH 3 Cl and CH 3 NH 2 . Which of t
ID: 806259 • Letter: C
Question
Consider a buffer solution consisting of CH3NH3Cl and CH3NH2. Which of the following statements are true concerning this solution?
(Ka for CH3NH3+ = 2.3 x 10 -11).
True False Adding more [CH3NH2] to the initial buffer solution will decrease the pH.
True False If [CH3NH3+] >[CH3NH2], then pH is larger than the pKa value.
True False If [CH3NH3+] > [CH3NH2], then the [H+] is smaller than the Ka value.
True False A solution consisting of 0.1 M CH3NH3Cl and 0.1 M CH3NH2 would be a more effective buffer than one containing 1 M CH3NH3Cl and 1 M CH3NH2
True False If [CH3NH3+] = [CH3NH2], then pH = 10.64.
True False If HCl were added to the initial buffer solution, then the [CH3NH3+] would decrease.
Explanation / Answer
The pH of a buffer solution can be calculated by using Henderson Hasselbalch equation.
The equation can be written as
pH = pKa + log [Base]/[Acid]
Th equilibrium reaction for buffer is
CH3NH2 + H2O <-------> CH3NH3+ + OH-
(Base) (acid)
pH = pKa + log [CH3NH2]/[CH3NH3+]
a)Adding more [CH3NH2] to the initial buffer solution will decrease the pH = False, because adding more [CH3NH2] will increase the value of log [CH3NH2]/[CH3NH3+]
b) If [CH3NH3+] >[CH3NH2], then pH is larger than the pKa value. = False
If [CH3NH3+] >[CH3NH2], then log [CH3NH2]/[CH3NH3+] would give a negative value, so pH will decrease
c) If [CH3NH3+] > [CH3NH2], then the [H+] is smaller than the Ka value. = False
As discussed above If [CH3NH3+] > [CH3NH2], then pH would be smaller than pKa but since pH & pKa are negative logarithms of H+ & Ka respectively, H+ would be greater than Ka
d) A solution consisting of 0.1 M CH3NH3Cl and 0.1 M CH3NH2 would be a more effective buffer than one containing 1 M CH3NH3Cl and 1 M CH3NH2 = False
More concentrated buffer would be more effective
e) If [CH3NH3+] = [CH3NH2], then pH = 10.64. = True
Ka for CH3NH3+ = 2.3 x 10 -11
pKa = - log (2.3 x 10^ -11) = 10.64
and when [CH3NH3+] = [CH3NH2], log term becomes 0, so pH = pKa
f) If HCl were added to the initial buffer solution, then the [CH3NH3+] would decrease. = False
If HCl is added, H+ ion from HCl would be taken up by CH3NH2 forming CH3NH3+, so conc. of CH3NH2 would decrease & that of CH3NH3+ would increase.
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