You are starting off with 180.0 mL of a buffered solution with a concentration o
ID: 959775 • Letter: Y
Question
You are starting off with 180.0 mL of a buffered solution with a concentration of H2PO4- of 0.0150 M and a concentration of HPO42- of 0.0350 M. The pKa of H2PO4- is 7.20. Calculate the initial pH of this buffered solution. How many millimoles of the acid and base are present in this buffer? Some amount of HCl is added, resulting in the new pH of the buffered solution to be 7.532. What is the ratio of the HPO42-/H2PO4- in the new solution? How many millimoles of HCl must have been added to your buffered solution?
Calculate the initial pH of this buffered solution?
____?___
How many millimoles of the acid and base are present in this buffer?
___?____ mmol HPO42- ______?____mmol H2PO4-
Some amount of HCl is added, resulting in the new pH of the buffered solution to be 7.532
What is the ratio of the HPO42-/H2PO4- in the new solution?
___?_____
How many millimoles of HCl must be added to your buffered solution?
___?____ mmol HCl
Explanation / Answer
1 ) initial pH = pKa + log [salt / acid]
pH = 7.20 + log (0.0350 /0.0150)
pH = 7.57
2)
millimoles of HPO4-2 = 180 x 0.0350 = 6.3
millimoles of H2PO4- = 180 x 0.0150 = 2.7
3)
new pH = 7.532
7.532 = 7.20 + log [HPO4- /H2PO4-]
[HPO4- /H2PO4-] = 2.15
4)
on addition of C mmol of HCl added to this buffer
pH = pKa + log [HPO4- - C /H2PO4- +C]
7.532 = 7.20 + log (6.3 -C / 2.7 + C)
6.3 -C / 2.7 + C = 2.15
C = 0.157
millimoles of HCl = 0.157
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