Question

Dissociation of phosphoric acid (₃P₀₄). Describe how you might prepare disodium phosphate ₂HPO²₄−) given a solution of phosphoric acid, 0.500 molar, and a solution of sodium hydroxide base, 0.500 molar.

a. Neutralize with HCl.
b. Combine with NaOH and filter.
c. React with NaHCO₃.
d. Precipitate with CaCl₂.

Answer 1

Disodium phosphate (Na₂HPO₄²⁻) is prepared by adding 0.500 M NaOH solution to 0.500 M phosphoric acid solution until reaching a pH of approximately 9.8. This corresponds to the second equivalence point in the titration curve, where phosphoric acid is transformed into disodium phosphate.

Step-by-step explanation:

To prepare disodium phosphate (₂HPO₄²⁻) from a solution of phosphoric acid (H₃PO₄) and a solution of sodium hydroxide (NaOH), both 0.500 molar, we can utilize a neutralization reaction. The appropriate reaction would involve adding sodium hydroxide to the phosphoric acid solution until the second equivalence point is reached, as indicated by the titration curve described in the reference material. At the second equivalence point, with a pH of approximately 9.8, the phosphoric acid is converted to disodium phosphate.

The correct step to prepare disodium phosphate is as follows:

1. Slowly add 0.500 M NaOH solution to the 0.500 M solution of phosphoric acid.
2. Monitor the pH of the solution and stop adding NaOH when the pH reaches about 9.8, which corresponds to the formation of disodium phosphate.
3. No filtration is necessary as there would be no precipitate formed during this reaction.

The other options mentioned, such as neutralizing with HCl, reacting with NaHCO₃, or precipitating with CaCl₂, are not relevant to the preparation of disodium phosphate from phosphoric acid and NaOH.