While preparing someone for surgery, Caralyn wants to ensure that her patient ha
ID: 534167 • Letter: W
Question
While preparing someone for surgery, Caralyn wants to ensure that her patient has the best chance of survival. She knows that blood is a buffered solution and wants to be able to replace any that is lost with a solution that is similar to pH to that of blood, which happens to be 7.35. She wants to prepare a solution using acetic acid and starts with a 1M solution of it. What is the pH of this solution? Propose something that could be used to make this buffer? How much of it would need to be in the solution to have it match the pH of the blood?
Explanation / Answer
Acetic acid (HAc) has Ka = 1.76*10-5.
Write down the dissociation of HAc as
HAc (aq) <=====> H+ (aq) + Ac- (aq)
initial 1.0 0.0 0.0
change -x +x +x
equilibrium (1.0 – x) x x
The acid dissociation constant is given as
Ka = [H+][Ac-]/[HAc] = (x).(x)/(1.0 – x)
===> 1.76*10-5 = x2/(1.0 – x)
Since Ka is small, we can assume x << 1.0 and write
1.76*10-5 = x2/1.0
===> x2 = 1.76*10-5
===> x = 4.195*10-3
Therefore, [H+] = 4.195*10-3 M and pH = -log [H+] = -log (4.195*10-3) = 2.377 2.38
Since the pH of 1.0 M acetic acid is much less than the pH of blood (7.35), hence acetic acid/acetate buffer is not a good choice.
A buffer must be so chosen than the pKa = -log Ka of the buffer must lie within 1 unit of the desired pH. The second ionization constant of phosphoric acid (H3PO4) is 7.21 and hence a good choice of the buffer will be the monosodium and disodium salts of phosphoric acid, viz, NaH2PO4 and Na2HPO4 (ans).
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