Estimate the pH of the resulting solution prepared by mixing 1.0 mole of solid disodium phosphate...

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Estimate the pH of the resulting solution prepared by mixing 1.0mole of solid disodium phosphate and 1.25 mole of hydrochloricacid. The pKas of phosphoric acid are 2.1,7.2,12.4. The answer is2.1

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3.8 Ratings (757 Votes)

first equilibrium

Na2HPO4 + HCl -----------------------------> NaH2PO4 + NaCl

1.0Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 1.25Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 0Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 0

0.0Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 0.25Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 1.0Â Â Â Â Â Â Â Â Â Â Â Â 1.0

second equilibrium

NaH2PO4 + HCl --------------------> H3PO4 + NaCl

1.0Â Â Â Â Â Â Â Â Â Â Â Â Â Â 0.25Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 0Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 0

0.75Â Â Â Â Â Â Â Â Â Â Â Â Â 0Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 0.25Â Â Â Â Â Â Â Â Â Â Â 0.25

pHÂ Â Â = pKa1 + log [NaH2PO4 / H3PO4 ]

pH = 2.1 + log (0.75 / 0.25)

pH = 2.58

2.58 is in between 2.1 to 7.2 . so answer is satisfied

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