Answer:
5.86
Explanation:
The equation of reaction is between methylamine and hydrochloric acid
CH₃NH₂ + HCl -------> CH₃NH⁺₃ + Cl⁻
At equivalence point, there is a mixture of equal volume of acid and base. As such, the concentration of CH₃NH⁺₃ will be half the initial starting concentration of the reactant.
i.e [CH₃NH⁺₃] =
= 0.095 M
Now, the dissociation of CH₃NH⁺₃ yields
CH₃NH⁺₃ ⇄ CH₃NH₂ + H⁺
The ICE Table can be constructed as follows:
CH₃NH⁺₃ ⇄ CH₃NH₂ + H⁺
Initial 0.095 0 0
Change -x x x
Equilibrium (0.095 - x) x x
------ equation (1)
We all know that:
and we are given = ; and ;
Then;
We can now re-write the equation (1) to be :
where a = 1; b = 2×10⁻¹¹; c = 1.9×10⁻¹²
Using the quadratic formula; ; we have
= OR
x =
[CH₃NH₂] = [H⁺] =
∴ pH = -log [H⁺]
pH = -log
pH = 5.86
Hence, the pH at the equivalence point for the titration = 5.86