A buffer solution contains 0.466 M K H 2 PO 4 and 0.349 M K 2 HPO 4 . Determine

Question

A buffer solution contains 0.466 M KH2PO4 and 0.349 M K2HPO4. Determine the pH change when 0.078 mol HBr is added to 1.00 L of the buffer.

pH change =

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A buffer solution contains 0.407 M CH3NH3Br and 0.291 M CH3NH2 (methylamine). Determine the pH change when 0.065 mol KOH is added to 1.00 L of the buffer.

pH after addition pH before addition = pH change =

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Determine the pH change when 0.092 mol HNO3 is added to 1.00 L of a buffer solution that is 0.461 M in HClO and 0.368 M in ClO-.

pH after addition pH before addition = pH change =

Explanation / Answer

Part A) pH before addition,

pH = pKa + log(base/acid)

     = 7.20 + log(0.349/0.466)

     = 7.074

After HBr = 0.078 mol added to 1 L buffer

new [KH2PO4] = (0.466 M x 1 L + 0.078 mol)/1 L = 0.544 M

new [K2HPO4] = (0.349 M x 1 L - 0.078 mol)/1 L = 0.271 M

pH = 7.20 + log(0.271/0.544) = 6.897

So, pH change = 6.897 - 7.074 = -0.177

Part B) pH before addition,

pH = pKa + log(base/acid)

     = 10.62 + log(0.291/0.407)

     = 10.474

After KOH = 0.065 mol added to 1 L buffer

new [CH3NH2Br] = (0.407 M x 1 L - 0.065 mol)/1 L = 0.342 M

new [CH3NH2] = (0.291 M x 1 L + 0.065 mol)/1 L = 0.356 M

pH = 10.62 + log(0.356/0.342) = 10.637

So, pH change = 10.637 - 10.474 = 0.163

Part C) pH before addition,

pH = pKa + log(base/acid)

     = 7.54 + log(0.368/0.461)

     = 7.442

After HNO3 = 0.092 mol added to 1 L buffer

new [HClO] = (0.461 M x 1 L + 0.092 mol)/1 L = 0.553 M

new [NaClO] = (0.461 M x 1 L - 0.092 mol)/1 L = 0.369 M

pH = 7.54 + log(0.369/0.553) = 7.364

So, pH change = 7.364 - 7.442 = -0.078

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