Calculate the pH of a buffer that is 0.10 M in KH2PO4 and 0.050 M in...

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Calculate the pH of a buffer that is 0.10 M in KH2PO4 and 0.050M in Na2HPO4. What is the pH after adding 5.0 mL of 0.20 M HCl to0.10 L of this buffer. The Ka value for H2PO4â€“ is 6.32 Ã— 10â€“8 or apKa of 7.199

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4.1 Ratings (589 Votes)

pH = pKa + log (base)/(acid)
base is 0.050M Na₂HPO₄
acid is 0.10 M KH₂PO₄

pKa = 7.199

pH = 7.199 + log (0.05)/(0.1)

pH = 6.89

100.0 mL x 0.050 = 5 mmols base Na₂HPO₄
100.0 mL x 0.10 = 10 mmols acid KH₂PO₄
added 5 mL x 0.20 M HCl = 1.0 mmols

. HPO₄^- Â Â + H⁺ ==> H₂PO₄^-

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 InitialÂ 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 5Â Â Â Â Â Â Â Â Â Â Â Â 0Â Â Â Â Â Â Â Â Â Â Â Â 10

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 AddÂ 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 Â Â Â Â Â Â Â Â Â 1.0

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 Change -1Â Â Â Â Â Â Â Â Â Â Â -1Â Â Â Â Â Â Â Â Â Â Â +1

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 EquilibriumÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 4Â Â Â Â Â Â Â Â Â Â Â Â 0Â Â Â Â Â Â Â Â Â Â Â Â 11

pH = pKa2 + log (base)/(acid)

base is 4 mmols Na₂HPO₄

acid is 11 mmol KH₂PO₄

pH = 7.199 + log 4/11

pH = 6.76

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