Which of the following combinations would be the best choice to buffer the pH of

Question

Which of the following combinations would be the best choice to buffer the pH of a solution at approximately 7? lonization Constants for Aqueous Weak Acids at 25 °c Acid Ka (pK.) 13 x 10 (2.89) 1.8 x 10 (4.74) Weak Acid Conjugate Base Hydrogen phthalate ion, CsH4(CO2H)(CO2) Acetate ion, CH, CO2 Phthalic acid, CeHs (CO2H)2 Acetic acid, CH3 CO2H Dihydrogen phosphate ion, H2PO4 Hydrogen phosphate ion. HPO ONaH2PO4 and Na2HPO O Nag HP04 and Na3 PO4 OR:Po, and NaH2PO4 Hydrogen phosphate ion, HPO 6.2x 10 8 (7.21) Phosphate ion, PO4 3.6 x 103 (12.44)

Explanation / Answer

To calculate the pH of a buffer solution, use Handersons-Hesselbalch equation i.e.,

pH = pKa + log [salt]/[acid]

Substitute the gives values for each weak acid and its salt concentrations and pKa values in the above equation to get the respective pH values.

For pthalic acid:

pH = 2.89 + log [hydrogen phthalate ion]/[phthalic acid]

So approximately it will be in between 3

Similarly for acetic acid pH will be nearly 5

For dihydrogen phosphate ion, pH will be nearly 7.

For hydrogen phosphate ion,pH will be nearly 12.

So among the given options,

First option NaH2PO4 and Na2HPO4 is the correct option.

Since NaH2PO4 splits into Na+ and H2PO4-

and Na2HPO4 splits into 2Na+ and HPO42-

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