Answer:
The answers are in the explanation. The pH is 5.91
Explanation:
The CH3NH2 reacts with HCl as follows:
CH3NH2 + HCl → CH3NH3⁺ + Cl⁻
When 200mL of HCl are added, the moles of CH3NH2 and HCl are reacting completely producing CH3NH3+ and Cl-. That means the species present are:
no H+. All reacted
yes H2O. Because the water is present in the solutions of HCl and CH3NH2
yes Cl-. Is a product of the reaction
Yes CH3NH2. Is consumed in the reaction but comes from the equilibrium of CH3NH3+
yes CH3NH3+. Is the other product of the reaction. MAJOR SPECIES
When 300.00mL of HCl are added, 100mL are in excess:
yes H+. Is in excess: H+ + Cl- = HCl in water. MAJOR SPECIES. Determine the pH of the solution.
yes H2O. Is present because the reactants are diluted.
yes Cl-. Is a product of reaction and comes from HCl.
Yes CH3NH2. The reactant is over but comes from the equilibrium of CH3NH3+
yes no CH3NH3+. Yes. Is a product and remains despite HCl is in excess.
To find the pH:
At equivalence point the ion that determines pH is CH3NH3+. Its concentration is:
0.100L * (0.200mol/L) = 0.0200 moles / 0.300L = 0.0667M CH3NH3+
The equilibrium of CH3NH3+ is:
Ka = Kw/kb = 1x10-14/4.4x10-4 = 2.273x10-11 = [H+] [CH3NH2] / [CH3NH3+]
As both [H+] [CH3NH2] comes from the same equilibrium:
[H+] = [CH3NH2] = X
2.273x10-11 = [X] [X] / [0.0667M]
1.5159x10-12 = X²
X = 1.23x10-6M = [H+]
As pH = -log [H+]
pH = 5.91
