1. Tardigrade
  2. Question
  3. Chemistry
  4. A 40 ml sample of an aqueous solution of the methylamine at 25° C is titrated with 0.15 M HCl and the equivalence point is reached when 40 ml of the acid have been added. The pH at the equivalence point is (Given: K a for CH 3 NH 3+is 4 / 3 × 10-11 ).

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Q. A $40 \,ml$ sample of an aqueous solution of the methylamine at $25^{\circ} C$ is titrated with $0.15 \,M \,HCl$ and the equivalence point is reached when $40\, ml$ of the acid have been added. The $pH$ at the equivalence point is ____
(Given : $K _{ a }$ for $CH _{3} NH _{3}^{+}$is $4 / 3 \times 10^{-11}$ ).

Equilibrium

Solution:

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$K _{ a } =\frac{\left[ CH _{3} NH _{2}\right]\left[ H _{3} O ^{+}\right]}{\left[ CH _{3} NH _{3}^{+}\right]} $
$=\frac{\frac{6}{80} h ^{2}}{(1- h )} \simeq \frac{6}{80} h ^{2} $
$ h =\sqrt{\frac{ K _{ a } \times 80}{60}}$
$\left[ H _{3} O ^{+}\right] =\frac{6}{80} h $
$ \sqrt{ K _{ a } \times \frac{6}{80}} =\sqrt{\frac{4}{3} \times 10^{-11} \times \frac{6}{80}} $
$=\sqrt{10^{-12}} $
$\left[ H _{3} O ^{+}\right] =10^{-6} M $
$ pH =6.0 $