How many moles of H2PO4 and HPO4 would be needed to prepare 1.0 L of a 0.01M phosphate buffer with a pH of 6.82?
Use the Henderson-Hasselbalch equation.
pH = pKa + log (base)/(acid).
Thank you, but I guess what my question should have been is what is the acid in this question and what is the base. I would assume that both H2PO4 and HPO4 are acids and phosphate is the base but Im not sure.
In this case, H2PO4 and HPO4 are actually acting as both the acid and base components in the phosphate buffer system.
To clarify, H2PO4 is the acid, also called dihydrogen phosphate, while HPO4 is the base, also known as hydrogen phosphate. Together, they form the phosphate buffer system, which helps maintain a stable pH.
Now let's proceed with determining the number of moles of each component needed to prepare the buffer.
Step 1: Find the pKa value.
Given that pH = 6.82, we need to find the pKa value to use in the Henderson-Hasselbalch equation. The pKa value for the H2PO4 / HPO4 buffer system is typically around 7.2.
Step 2: Apply the Henderson-Hasselbalch equation.
pH = pKa + log (base)/(acid)
Substituting the given values:
6.82 = 7.2 + log (base)/(acid)
Step 3: Rearrange the equation to solve for (base)/(acid).
log (base)/(acid) = 6.82 - 7.2
log (base)/(acid) = -0.38
Step 4: Calculate (base)/(acid).
(base)/(acid) = 10^(-0.38)
(base)/(acid) = 0.453
This means the ratio of the base (HPO4) to the acid (H2PO4) is 0.453.
Step 5: Determine the moles of each component needed.
To prepare 1.0 L of a 0.01 M phosphate buffer, we need to determine the moles of each component required.
Let's assume x moles of H2PO4 are needed. Therefore, the moles of HPO4 needed would be (0.453x) since (base)/(acid) ratio is 0.453.
According to the molarity formula, M = moles/volume. Therefore:
0.01 M = x moles / 1.0 L
Solving for x, we find that:
x = 0.01 M x 1.0 L = 0.01 moles
Thus, we need 0.01 moles of H2PO4 and (0.453 * 0.01) = 0.00453 moles of HPO4 to prepare 1.0 L of a 0.01 M phosphate buffer with a pH of 6.82.
Please note that the final answer is rounded to the appropriate number of significant figures.