1. Determine the pH of the buffer solution that is prepared by mixing 745 mL of
ID: 1033518 • Letter: 1
Question
1. Determine the pH of the buffer solution that is prepared by mixing 745 mL of 0.950 M HEPES-H (CgH18N2O4S) to 47.8 mL of 1.70 M HEPES (CBH17N204S) Assume the 5% approximation is valid Report your answer to 2 decimal places ?? S03 HEPES-H The pKa of HEPES-H is 7.56 Submit Answer Tries 0/5 2. Determine the pH of the HEPES-H/HEPES buffer solution after the addition of 292 mL of 0.0217 M solution of hydroiodic acid to the existing buffer solution. Assume the 5% approximation is valid Submit Answer Tries 0/5Explanation / Answer
Answer:
pH of any buffer is given by Henderson-Hasselbalch equation as,
pH = pKa of Acid + log (mmol of Base/mmol of acid)
For HEPES-H(Acid) / HEPES (Conjugate base) buffer, pH is given as,
pH = pKa of (HEPES-H) + log (mmol of HEPES / mmol of HEPES-H) .......... (1)
From given molarity and corresponding volumes in mL we have,
mmol of HEPES-H = Molarity* volume in mL = 0.950*745.0 = 707.75 mmol
mmol of HEPES = Molarity* volume in mL = 1.70*47.8 = 81.26 mmol.
pKa of HEPES-H = 7.56
Using these values in eq.(1)
pH = 7.56 + log(81.26/707.75)
pH= 7.56 + (-0.94)
pH = 6.62
pH of the resultant buffer will b 6.62.
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2) Effect on Buffer pH on adding 292.0 mL of 0.0217 M HI on HEPES-H /HEPES buffer:
Mmol of HI i.e. H+ added = Molarity*Volume in mL = 0.0217*292.0 = 6.34 mmol.
Addition of HI an Acid will increase mmol of Acid component HEPES-H and that at the cost of mmol of HEPES.
Hence on the addition of HI,
mmol of HEPES-H = 707.75 + 6.34 = 714.09 mmol
mmol of HEPES = 81.26-6.34 = 74.92 mmol.
Then pH is given as,
pH = 7.56 + log (74.92/714.09)
pH = 7.56 + (-0.98)
pH = 6.58
Is the pH of the buffer on addition of HI
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