Question

Now assume you wish to make a buffer at the same pH, using the same substances, but want the total phosphate molarity ([HPO₄₂−]+[H₂PO₄]) to equal 0.25 M. What concentration of the Na₂HPO₄would be required?

Express your answer to two significant figures.

Answer 1

To calculate the required concentration of Na₂HPO₄ in a buffer solution with a total phosphate concentration of 0.25 M, use the Henderson-Hasselbalch equation and the desired pH to determine the ratio of [A-] to [HA]. Then allocate the total phosphate concentration between NaH₂PO₄ and Na₂HPO₄ accordingly, using their molar ratios.

Step-by-step explanation:

To create a buffer with a total phosphate molarity (total phosphate) of 0.25 M using NaH₂PO₄ (sodium dihydrogen phosphate) and Na₂HPO₄ (disodium hydrogen phosphate), you need to apply the Henderson-Hasselbalch equation:

pH = pKa + log ([A-]/[HA])

Where:

• pH is the pH of the buffer solution
• pKa is the negative logarithm of the acid dissociation constant
• [A-] is the concentration of the base form (here, Na₂HPO₄)
• [HA] is the concentration of the acid form (here, NaH₂PO₄)

The desired buffer pH determines the ratio of [A-] to [HA]. Once the ratio is known from the Henderson-Hasselbalch equation, you can use the given total phosphate concentration to find the individual concentrations of NaH₂PO₄ and Na₂HPO₄. Note that the pKa value applicable for the acid/base pair must be used to calculate the pH. The exact concentrations also depend on the base's pKa and the system's pH.

The answer will be expressed to two significant figures as required.