1. Determine the pH of the buffer solution that is prepared by mixing 259 mL of
ID: 928220 • Letter: 1
Question
1. Determine the pH of the buffer solution that is prepared by mixing 259 mL of 0.0983 M BISTRIS propaneH (C11H27N2O6+ ) to 855 mL of 0.0729 M BIS-TRIS_propane (C11H26N2O6) ). Assume the 5% approximation is valid. Report your answer to 2 decimal places. The pKa of BISTRIS propaneH is 9.10. Submit Answer Tries 0/3 2. Determine the pH of the BISTRIS propaneH/BISTRIS propane buffer solution after the addition of 25.2 mL of 0.643 M solution of rubidium hydroxide to the existing buffer solution. Assume the 5% approximation is valid.
Explanation / Answer
1) The given buffer is a basic buffer comprising of a weak base Bis-trispropane and its conjugate acid bis-trispropane-H+
pKa of conjuagte acid is 9.1. Thus pKb of base is 14-9.1 = 4.9
pOH of basic buffer is given by Hendersen equation as
pOH = pKb + log [conjugate acid]/[base]
substituting the given values
pOH = 4.9 + log [259x0.0983/1114]/[855x0.0729/1114] = 4.511
Thus pH of this buffer is 14- 4.511 = 9.489
2) When a base is added to this buffer the reaction is
BH+ + OH- ------> B + H2O
259x 0.0983 25.2 x0.643 855 x 0.0729 Initial concentrations
9.2561 0.0 78.5331 after reaction
9.2561/1139.2 78.5331/1139.2 molarity after reaction
Substituting in Hendersen equation
pOH = 4.9 + log 9.2561/78.5331 = 3.9712
and pH = 14- 3.9712 = 10.02
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