Question

Calculate the ratio of ch3nh2 to ch3nh3cl required to create a buffer with ph = 10.22.

Answer 1

When the  Ka value for CH3NH2 = 2.27 x 10 ^-11

and when we have the Ka value so, we can get Pka from this formula:

Pka = -㏒Ka

∴Pka = -㏒ (2.27 x10^-11)
         = 10.6 

now, to get the base/acid ratio by using Henderson-Hasselblach equation:

PH = Pka + ㏒[base/acid]

so by substitution: 

10.22 = 10.6 + ㏒[base/acid]

∴[base/acid] = 0.417
 
∴[CH3NH2/CH3NH3Cl] = 0.417