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A buffer solution has 0.625 M HCN and 0.575 M CN−. If 0.030 moles of NaOH is added to 400. mL of the buffer solution, what is the pH after the addition? The pKa of hydrocyanic acid is 9.31.

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Answer:

pH = 9.393

Explanation:

Give that 400mL buffer 1L = 1000mL 0.625 mol of HCN solution = 0.250 mol HCN

Therefore,

400mL buffer solution 1L = 1000mL 0.575 mol CN^- solution = 0.230 mol of CN^-

Pka HCN = 9.31

Therefore, to get ph:

pH = 9.32 + log(0.230 mol + 0.030 mol)/ (0.250 mol - 0.030 mol)

pH = 9.32 + 0.0726

pH = 9.393

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