Question

You dissolve 0.735 g of salt cyclphexylamine chloride with enough water in a 100.0 ml volumetric flask. You then titrate the solution with 0.050 M sodium hydroxide. Ka of cyclohexylamine is 2.71E-11.
a) calculate NaOH volume needed of reach the equivalent point.
b) calculate the pH at the equivalent point.

Answer 1

MW of cyclohexylamine chloride is 134.5 g

So,

Moles of this weak acid taken = Mass/MW = 0.735/134.5 = 0.00546

Since this is a monobasic acid, so it reacts in a 1:1 molar ratio with NaOH

So,

(a)

Moles of NaOH needed = Moles of acid present = 0.00546

So,

Volume of base needed = Moles/Molarity = 0.00546/0.050 = 0.1092 L = 109.2 mL

(b)

Lets denote the acid by HA

So, the reaction taking place at equivalent point is:

A^- + H₂O ---> HA + OH^-

Initial (0.00546/209.2) 0 0

Eqb (0.00546/0.2092)-x x x

K_b = ([HA]*[OH^-])/[A^-] = (x^2)/((0.00546/0.2092)-x) = (10^-14)/(2.71*(10^-11))

Solving we get:

x = [OH^-] = 0.00292 M

So,

pOH = -log([OH^-]) = 2.53

So,

pH = 14-2.53 = 11.47

Hope this helps !