Question

How many milliliters of 1.96 MNaOH must be added to 3.00x10² mL of 0.14 MH₂PO⁻⁴ to make a buffer solution with a pH of 6.90? Round your answer to 2 significant digits. Note: Reference the pKa of weak acids at 25°C table for additional information.

Answer 1

Without the pKa value of H2PO4-, a specific calculation cannot be completed for the volume of 1.96 M NaOH needed. The general approach would involve using the Henderson-Hasselbalch equation and stoichiometry to determine the needed volume of NaOH.

Step-by-step explanation:

The question asks how many milliliters of 1.96 M NaOH must be added to 300 mL of 0.14 M H2PO4− to create a buffer with a pH of 6.90. This involves using the Henderson-Hasselbalch equation to calculate the ratio of conjugate base to acid needed for the desired pH and then determining the amount of NaOH required to convert the necessary amount of H2PO4− into its conjugate base. Unfortunately, without the pKa value of H2PO4, the specific calculation cannot be completed. For a buffer system, the equation pH = pKa + log([A−]/[HA]) is typically used, where [A−] is the concentration of the conjugate base and [HA] is the concentration of the acid.

Since the pKa is not provided, a general approach involves rearranging the Henderson-Hasselbalch equation to solve for the ratio [A−]/[HA], and then using stoichiometry to calculate the volume of NaOH needed to achieve that ratio, considering the molarity of the NaOH solution provided. To illustrate, if using a hypothetical pKa of H2PO4, the volume of NaOH could be determined after computing the moles of H2PO4 present in the solution and the desired moles of conjugate base necessary.