When a solution contains a weak acid and its conjugate base or a weak base and i

Question

When a solution contains a weak acid and its conjugate base or a weak base and its conjugate acid, it will be a buffer solution. Buffers resist change in pH following the addition of acid or base. A buffer solution prepared from a weak acid (HA) and its conjugate base (A ) is represented as HA(aq) H+ (aq) + A- (aq) The buffer will follow Le Chatelier's principle. If acid is added, the reaction shifts to consume the added H^+, forming more HA. When base is added, the base will react with H^+, reducing its concentration. The reaction then shifts to replace H^+ through the dissociation of HA into H^+ and A^- . In both instances, [H ^+] tends to remain constant. The pH of a buffer is calculated by using the Henderson-Hasselbalch equation: PH=pk_a+log|A^-|/|HS| What is the pH of a buffer prepared by adding 0.506 mol of the weak acid HA to 0.507 mol of NaA in 2.00 L of solution? The dissociation constant KA of HA is 5.66 x 10 ' . Express the pH numerically to three decimal places. Submit Hints My Answers Give Up Review Part Part B What is the pH after 0.150 mol of HC1 is added to the buffer from Part A? Assume no volume change on the addition of the acid. Express the pH numerically to three decimal places. What is the pH after 0.195 mol of NaOH is added to the buffer from Part A? Assume no volume change on the addition of the base. Express the pH numerically to three decimal places.

Explanation / Answer

A) pka = -log Ka = -log ( 5.66 x 10^ -7) = 6.247

pH = pka + log [A-]/[HA]

pH = 6.247 + log ( 0.507/2) / ( 0.506/2)

      = 6.248

B) HCl gives H+ which reacts with A- to give HA

H+ moles added = 0.15 ,

A- moles = 0.507-0.15 = 0.357 ,    HA moles = 0.506+0.15 = 0.656

pH = 6.247+log ( 0.357/2) / ( 0.656/2)

       = 5.983

C) NaOH gives OH- moles which react with HA to give A-

hence HA moles after reacting with 0.195 moles OH- = 0.5060-0.195 = 0.311

A- moles = 0.507+0.195 = 0.702

pH = 6.247+log ( 0.702/2) / ( 0.311/2)

        = 6.601

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