Multiple Choice
A 20.00-mL sample of 0.150 M NH3 is being titrated with 0.200 M HCl. What is the pH after 15.00 mL of HCl has been added? Kb of NH3 = 1.8 × 10⁻⁵
224
views
18. Aqueous Equilibrium
Titrations: Weak Base-Strong Acid
Multiple Choice
Consider the titration of a 25.0-mL sample of 0.175 M CH3NH2 with 0.150 M HBr. What is the initial pH of the CH3NH2 solution before any HBr is added?
A
2.78
B
11.22
C
9.25
D
7.00
Verified step by step guidance1
Identify the chemical species involved: CH3NH2 is a weak base, and we need to find its initial pH before any HBr is added.
Write the equilibrium expression for the weak base CH3NH2 in water: CH3NH2 + H2O ⇌ CH3NH3+ + OH−.
Use the base dissociation constant (Kb) for CH3NH2 to set up the expression: Kb = [CH3NH3+][OH−] / [CH3NH2].
Assume that the initial concentration of CH3NH2 is 0.175 M and that the change in concentration due to dissociation is small, so [CH3NH2] remains approximately 0.175 M.
Calculate the concentration of OH− using the Kb value and the initial concentration of CH3NH2, then find the pOH and convert it to pH using the relation: pH = 14 - pOH.
Related Videos
Related Practice
