You are to prepare a KH2PO4 and Na2HPO4 solution with pH=7.31.
What should the concentration of [2(HPO4) 2-] / [H2PO4-]
HA + H2O---> H3O+ + A-
Ka=[H3O+][A-]/[HA]
..........HA I H3O+ I A-
I.........I 0.1 M I 0 I 0 I
C........I-2(0.1*0.025) I (0.1*0.025)I (0.1*0.025) I
E.........I 0.1-2(0.1*0.025) I (0.1*0.025) I (0.1*0.025)
Ka=[(0.1*0.025][(0.1*0.025]/[0.1-2(0.1*0.025]
pka=-log(ka)
****This is just the mole ratio concentration. To determine an exact concentration of each you need to know the concentration that you want. What molarity?
I apologize, I am pasting from a word document. before I upload what I think and I posted the wrong one. Ignore original post.
Use the henderson-hasselbalch equation and solve for the ratio.
To determine the concentration of [2(HPO4)2-] / [H2PO4-], we first need to calculate the concentrations of KH2PO4 and Na2HPO4 required to achieve a pH of 7.31.
Here's how you can approach this:
Step 1: Understand the dissociation of KH2PO4 and Na2HPO4
KH2PO4 dissociates as follows:
KH2PO4 ↔ K+ + H2PO4-
Na2HPO4 dissociates as follows:
Na2HPO4 ↔ 2Na+ + HPO42-
Step 2: Determine the equilibrium expression for the phosphate buffer system
The pH of a phosphate buffer system is determined by the equilibrium between the dihydrogen phosphate ion (H2PO4-) and the hydrogen phosphate ion (HPO42-):
H2PO4- ↔ H+ + HPO42-
Step 3: Use the Henderson-Hasselbalch equation
The Henderson-Hasselbalch equation is given by:
pH = pKa + log([A-] / [HA])
For the phosphate buffer system, since H2PO4- acts as the acid (HA) and HPO42- acts as the conjugate base (A-), we can write:
pH = pKa + log([HPO42-] / [H2PO4-])
We know that pH = 7.31, so we need to find the corresponding ratio [HPO42-] / [H2PO4-] to achieve this pH.
Step 4: Calculate the ratio [HPO42-] / [H2PO4-]
Since we want pH = 7.31, we need to consider the pKa value of H2PO4- to substitute into the Henderson-Hasselbalch equation. The pKa of H2PO4- is around 7.21.
Therefore, we have:
7.31 = 7.21 + log([HPO42-] / [H2PO4-])
Simplifying the equation gives:
0.1 = log([HPO42-] / [H2PO4-])
Step 5: Convert the logarithmic equation to exponential form
By converting the equation to exponential form, it becomes:
[HPO42-] / [H2PO4-] = 10^0.1
Calculating the value gives:
[HPO42-] / [H2PO4-] ≈ 1.2589
So, the concentration of [2(HPO4)2-] / [H2PO4-] is approximately 1.2589.
Note: The exact concentration of KH2PO4 and Na2HPO4 needed to achieve a pH of 7.31 depends on the initial concentrations of these compounds and the buffer capacity of the system. This calculation assumes ideal buffer behavior.