analytical chemistry

A solution containing 0.40 M B (a weak base) and 0.20 M BH+ (the salt of the weak
base) has a pH = 10.30. What is the pH after 25 mmol of HCl is added to 250 mL of this
solution?

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asked by axy
  1. First use the Henderson-Hasselbalch equation and solve for pKa.
    pH = pKa + log (base)/(acid)
    10.30 = pKa + log [0.4/0.2]
    pKa = ?

    mmols B initially = 0.4 x 250 = 100
    mmols BH^+ initially = 0.2 x 250 = 50

    ..........B + H^+ ==> BH^+
    I........100...0......50
    add............25..........
    C........-25...-25....+25
    E.........75...0.......75

    Using pKa from above, substitute the E line into a new HH equation and solve for pH.
    Since B = BH^+ then the log term will be zero and the new pH will equal pKa.

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    posted by minshee

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