Answer:
We will need 147.772 mL of KH2PO4 to make this solution
Explanation:
For this case we can give the following equation:
H2PO4 - ⇄ H+ + HPO42-
With following pH- equation:
pH = pKa + log [HPO42-]/[H2PO4-]
7.05 = 7.21 + log [HPO42-]/[H2PO4-]
-0.16 = log [HPO42-]/[H2PO4-]
10^-0.16 = [HPO42-]/[H2PO4-]
0.6918 = [HPO42-]/[H2PO4-]
Let's say the volume of HPO42-= x then the volume of H2PO4- will be 250 mL - x
Since both have a concentration of 1M = 1 mol /L
If we plug this in the equation 0.6918 = [HPO42-]/[H2PO4-]
0.6918 = x / (250 - x)
0.6918*250 - 0.6918x = x
172.95 = 1.6918x
x = 102.228 mL
The volume of HPO42- = 102.228 mL
Then the volume of H2PO4- = 250 - 102.228 = 147.772 mL
To control this we can plug this in the pH equation
7.05 = 7.21 + log [HPO42-]/[H2PO4-]
7.05= 7.21 + log (102.228 / 147.772) = 7.05
We will need 147.772 mL of KH2PO4 to make this solution
