Question

Calculate the pH when 30.0 mL of 0.200 M HBr is mixed with 30.0 mL of 0.400 M CH₃NH₂ (Kb = 4.4 × 10⁻⁴).

Answer 1

Given that, Concentration of HBr = 0.200 M Volume of HBr = 30.0 mL Concentration of CH NH, = 0.400 M Volume of CH NH, = 30.0mL K = 4.4x104 The reaction when HBr is mixed with CH NH, is given as follows: Millimoles Initial Change Equilibrium CH3NH2 + HBr + CH NH; +Br 12.0 6.0 0 0 -6.0 - 6.0 +6.0 0 +6. 0 +6.0 Calculate the number of millimoles of CH NH, and HBr added as follows: Number of millimolesof CH NH, = Concentration x Volume =0.400 MX 30.0 mL = 12.0 millimoles Number of millimoles of HBr = Concentration x Volume =0.200 MX 30.0 mL = 6.0 millimoles Total volume = 30 mL + 30 mL=60.0 mL

K, of CH NH, = 4.4 x 10-4 PK, =-log K, = -log(4.4x104) = 3.357 Calculate pOH of solution using Henderson-Hasselbalch equation as follows: pOH = pK, +108 CH,NH,) ) (TCH NH] pOH = 3.357 +10g (6:0) 1 pOH = 3.357 Calculate the molarity of HBr after addition as follows: millimoles Molarity of H* after addition = Total volume 6.0 millimoles 60.0 mL 0.1M Calculate pH of solution as follows: pH = 14 - POH = 14 - 3.357 = 10.643

Therefore, the pH of solution using the given data is 10.64.