What concentrations of acetic acid (pKa = 4.76) and acetate would be required to

Question

What concentrations of acetic acid (pKa = 4.76) and acetate would be required to prepare a 0.20 M buffer solution at pH 4.9? Note that the concentration and/or pH value may differ from that in the first question. STRATEGY Rearrange the Henderson-Hasselbalch equation to solve for the ratio of base (acetate) to acid (acetic acid), [A^-]/[HA]. Use the mole fraction of acetate to calculate the concentration of acetate. Calculate the concentration of acetic acid. Step 1: Rearrange the Henderson Hasselbalsh equation to solve for [A^-]/[HA] The Henderson-Hasselbalsh equation is pH = pK_a + log([A^-]/[HA]) Use the Henderson-Hasselbalsh equation to solve for [A^-]/[HA] if the solution is at pH 4.5. [A^-]/[HA] = ter your answer as a number. For example, 9.6. Do not enter a formula like 10^6 + 2.

Explanation / Answer

According to HendersonHasselbalch equation

Ph = Pka + log{[A-]/[HA]}

given

Ph = 4.5 , [A-] = 0.10 M , Pka = 4.76

now

4.5 = 4.76 + log(0.1/HA)

-0.26 = log(0.1/HA)

0.1/ [HA] = 0.549

[HA] = [aceticacid] = 0.18 M

  

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