Question

Consider a buffered solution containing CH3NH3Cl and CH3NH2.

Which of the following statements are true concerning this solution? (Ka for CH3NH3+ = 2.2 × 10-11). (Select all that apply.)

If [CH3NH2] > [CH3NH3+], then pH is larger than the pKa value.

If NaOH were added to the initial buffered solution, then the [CH3NH2] would increase.

If [CH3NH2] = [CH3NH3+], then pH = 3.36

If [CH3NH2] < [CH3NH3+], then pH is larger than the pKa value.

A solution consisting of 2.5 M CH3NH3Cl and 2.5 M CH3NH2 would have a higher capacity than one containing 0.5 M CH3NH3Cl and 0.5 M CH3NH2.

Answer 1

pH = pK_a + log([CH₃NH₂]/[CH₃NH₃⁺])

pK_a = -logK_a = -log(2.2*10^-11)

pK_a = 10.65

a. If [CH₃NH₂] > [CH₃NH₃⁺] , then [CH₃NH₂]/[CH₃NH₃⁺]​ > 1 and log([CH₃NH₂]/[CH₃NH₃⁺])​ = +ve.

And pH > pK_a.

True

b. If NaOH is added to the initial buffered solution, then the [CH₃NH₂] would increase.​

CH₃NH₃⁺ + OH^- => CH₃NH₂ + H₂O

Adding NaOH => CH₃NH₂ is formed => [CH₃NH₂] increases.

True

c. If [CH₃NH₂] = [CH₃NH₃⁺], then log([CH₃NH₂]/[CH₃NH₃⁺]) = log(1) = 0

pH = pK_a

pH = 10.65

False

d. If [CH₃NH₂] < [CH₃NH₃⁺] , then [CH₃NH₂]/[CH₃NH₃⁺]​ < 1 and log([CH₃NH₂]/[CH₃NH₃⁺])​ = -ve.

And pH < pK_a.

False

e. Higher concentrations give more effective buffer.

So, a solution consisting of 2.5 M CH₃NH₃Cl and 2.5 M CH₃NH₂ would have a higher capacity than one containing 0.5 M CH₃NH₃Cl and 0.5 M CH₃NH₂.

True