In a solution of 96 microM sodium phosphate buffer (this value represents all fo
ID: 1004514 • Letter: I
Question
In a solution of 96 microM sodium phosphate buffer (this value represents all forms of phosphate no matter the ionization state) at a pH 7.2, what is the ratio of HPO4 2- to H2PO4- ? The pKas of phosphoric acid are 2.15, 6.86, and 12.32.If someone could help walk me through how to solve this problem, rather than give me the answer, it would be super helpful. I keep getting wrong values and cannot figure out the right way to solve this problem.
Also could someone explain what the information in parenthesis means? Is the concentration given just extra information to throw me off?
Thank you!! In a solution of 96 microM sodium phosphate buffer (this value represents all forms of phosphate no matter the ionization state) at a pH 7.2, what is the ratio of HPO4 2- to H2PO4- ? The pKas of phosphoric acid are 2.15, 6.86, and 12.32.
If someone could help walk me through how to solve this problem, rather than give me the answer, it would be super helpful. I keep getting wrong values and cannot figure out the right way to solve this problem.
Also could someone explain what the information in parenthesis means? Is the concentration given just extra information to throw me off?
Thank you!!
If someone could help walk me through how to solve this problem, rather than give me the answer, it would be super helpful. I keep getting wrong values and cannot figure out the right way to solve this problem.
Also could someone explain what the information in parenthesis means? Is the concentration given just extra information to throw me off?
Thank you!!
Explanation / Answer
According to the henderson hasselbalch equation:
pH = pKa + Log [conjugate base]/[weak acid]
pH = 7.2
pKa = is the disassociation constant for the weak acid
[conjugate base] = the concnetration of the conjugate base in the solution. A conjugate base is the part left over when the weak acid you are using has given up a H+ ion. If you are making a phosphate buffer your weak acid is H2PO4- ion (monobasic).
H2PO4-(aq) => H+(aq) + HPO4 2- (aq)
(Weak acid) (conjugate base)
Note: The phosphate ion if it was fully protonated would be H3PO4 (phosphoric acid). In other words it is a triprotic acid since it can donate 3 H+ ions. The conjugate base of phosphoric acid is PO4 3-. This conjugate base is called tribasic since it can accept up to three H+ ions. If it is already partially protonated with one H+ ion you would have HPO42-. This is now a dibasic phosphate ion since it already has one hydrogen so therefore can only accept two more.
H2PO4- is monobasic since it already has two H+ ions so can only accept one more.
pH = pKa + Log [conjugate base]/[weak acid]
pH = pKa + Log [HPO4 2-]/[H2PO4- ]
pH = 7.2
The pKa of acid (H2PO4-) is pKa = 6.86
As H3PO4 is triprotic so three values of pKa i.e 2.12 for H3PO4, 6.86 for H2PO4- and 12.32 for HPO42-
7 = 6.86 + Log [HPO4 2-]/[H2PO4-]
7 – 6.86 = Log HPO4 2-]/[H2PO4-]
0.140 = Log HPO4 2-]/[H2PO4-] ………………………taking the anti log of both sides:
1.38= [HPO4 2-]/[H2PO4-] .....................ratio
The number of moles in 1L solution is the number of moles of phosphate, which is the sum of both your phosphate salts:
[HPO4 2-] + [H2PO4-] = 96 microM