You are asked to prepare 500. mL of a 0.300 M acetate buffer at pH 5.10 using on

Question

You are asked to prepare 500. mL of a 0.300 M acetate buffer at pH 5.10 using only pure acetic acid (MW=60.05 g/mol, pKa = 4.76), 3.00 M NaOH, and water. Answer the following questions regarding the preparation of the buffer. How many grams of acetic acid will you need to prepare the 500 mL buffer? Note that the given concentration of acetate refers to the concentration of all acetate species in solution. What volume of 3.00 M NaOH must you add to the acetic acid to achieve a buffer with a pH of 5.10 at a final volume of 500 mL? Ignore activity coefficients.) Pour the beaker contents into a 500 mL volumetric flask. Rinse the beaker several times and add the rinses to the flask. Swirl the flask to mix the solution. Add water to the mark to make the volume 500 mL. Stopper the flask and invert how many times to ensure complete mixing?

Explanation / Answer

1)

total moles of buffer = 500 x 0.3 / 1000 = 0.15

pH = pKa + log [salt / acid]

5.10 = 4.76 + log [acetate / acetic acid]

[acetate / acetic acid] = 2.188

acetate + acetic acid = 0.15

2.188 acetic acid + acetic acid = 0.15

moles of acetic acid = 0.04706

mass of acetic acid = 0.04706 x 60.05

mass of acetic acid = 2.83 g

2)

moles of acetate = 0.1030

moles of NaOH = 0.1030

moles = molarity x volume

0.1030 = 3 x V

Volume = 0.0343 L

volume of NaOH = 34.3 mL

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