Question

Is the pH at the equivalence point in the titration of 0.10 M (CH3)2NH with 0.10 M HCl acidic, basic, or neutral

Answer 1

Dimethylamine is a weak base and HCl is a strong acid.

they react as :

(CH₃)₂NH + HCl    (CH₃)₂NH2⁺      +             Cl^-

Kb for (CH₃)₂NH = 7.4 x 10^-4 at 25 C

pKb = 3.13

so 0.10 M HCl will react with 0.10 M moles of (CH₃)₂NH to produce (0.10 / 2 = ) 0.05 M of (CH₃)₂NH2⁺

so we cannot use buffer equation as above as now no (CH₃)₂NH is left and all (CH₃)₂NH is converted into (CH₃)₂NH2⁺.

using hydrolisis equation for a salt of weak base and strong acid ...

pH = 1/2 [pKw - pKb - log c ]

pH = 1/2 [ 14 – 3.13 - log 0.05 ]

pH = 1/2 [ 10.87 - 1.3 ]

pH = 1/2 [ 9.57 ]

pH = 4.785

It will be acidic.