Consider a buffer solution consisting of CH 3 NH 3 Cl and CH 3 NH 2 . Which of t
ID: 831837 • 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 A solution consisting of 0.25 M CH3NH3Cl and 0.25 M CH3NH2 would be a more effective buffer than one containing 2.5 M CH3NH3Cl and 2.5 M CH3NH2
True/ False If [CH3NH3+] > [CH3NH2], then the [H+] is smaller than the Ka value.
True False Adding more [CH3NH3+] to the initial buffer solution will decrease the pH.
True/ False If HCl were added to the initial buffer solution, then the [CH3NH3+] would increase.
True/ False If [CH3NH3+] = [CH3NH2], then pH = 7.00.
True/ False If [CH3NH3+] >[CH3NH2], then pH is larger than the pKa value.
Explanation / Answer
False: A solution consisting of 0.25 M CH3NH3Cl and 0.25 M CH3NH2 would be a more effective buffer than one containing 2.5 M CH3NH3Cl and 2.5 M CH3NH2
Lower concentrations give a less effective buffer
False: If [CH3NH3+] > [CH3NH2], then the [H+] is smaller than the Ka value.
[CH3NH3+] >[CH3NH2] => [CH3NH2]/[CH3NH3+] < 1 => log([CH3NH2]/[CH3NH3+]) < 0
=> pH = pKa + log([CH3NH2]/[CH3NH3+]) < pKa
=> 10^(-pH) > 10^(pKa)
=> [H+] > Ka
True: Adding more [CH3NH3+] to the initial buffer solution will decrease the pH.
pH = pKa + log([CH3NH2]/[CH3NH3+])
[CH3NH3+] increases => pH decreases
True: If HCl were added to the initial buffer solution, then the [CH3NH3+] would increase.
HCl + CH3NH2 => CH3NH3+ + Cl-
Adding HCl => CH3NH3+ is formed => [CH3NH3+] increases
False: If [CH3NH3+] = [CH3NH2], then pH = 7.00.
pH = pKa + log([CH3NH2]/[CH3NH3+]) = -log Ka + log([CH3NH2]/[CH3NH3+])
= -log(2.3 x 10^(-11)) + log(1) = 10.64
False: If [CH3NH3+] >[CH3NH2], then pH is larger than the pKa value.
[CH3NH3+] >[CH3NH2] => [CH3NH2]/[CH3NH3+] < 1 => log([CH3NH2]/[CH3NH3+]) < 0
=> pH = pKa + log([CH3NH2]/[CH3NH3+]) < pKa
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