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 0.10 M buffer solution at pH 4.6? Note that the concentration and/or pH value may differ from that in the first question. STRATEGY 1. Rearrange the Henderson-Hasselbalch equation to solve for the ratio of base (acetate) to acid (acetic acid), [A–]/[HA]. 2. Use the mole fraction of acetate to calculate the concentration of acetate. 3. Calculate the concentration of acetic acid. Step 1: The ratio of base to acid is 0.69. Step 2: The mole fraction of acetate is 0.41, and the concentration of acetate is 0.041. Step 3: Calculate the concentration of acetic acid.

Explanation / Answer

pH= pKa + log [acetate]/[acetic acid]; 4.60 = 4.76 + log [acetae]/[acetic acid] ; log [acetate]/[acetic acid]= -0.16 ;

[acetate]/[acetic acid] =0.69

add 1 to left and right hand side. thus [acetate] + [acetic acid] = 1.69 [acetic acid]

so mole fraction of acitic acid is [acetic acid]/[acetate] + [acetic acid] = 0.59 so mole fraction of acetate =0.41

total concentration is 0.10 ; so concentration of acetate is 0.41*0.10 = 0.041 M.

concentration of Acetic acid is 0.59*0.10 = 0.059 M.

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