You are asked to prepare 500. mL of a 0.300 M acetate buffer at pH 5.00 using on

Question

You are asked to prepare 500. mL of a 0.300 M acetate buffer at pH 5.00 using only pure acetic acid MW=60.05 g/mol, pK=4.76), 3.00 M NaOH, and water. Answer the following questions regarding the preparation of the buffer. 1. 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. Number 9.01 2. What volume of 3.00 M NaOH must you add to the acetic acid to achieve a buffer with a pH of 5.00 at a final volume of 500 mL? (ignore activity coefficients.) Number 11.0 mL 3. 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? Incorrect. O 20

Explanation / Answer

Q2

volume of NaOH for pH = 5, given V = 500 mL

pH = pKa + log(a-/HA)

5 = 4.76 + log(A-/HA)

A-/HA = 10^(5-4.76) = 1.73780

mmol of acetate = 0.500*0.3 = 0.15

after adding "x" mol of NaOH..

mol of Acetic acid left = 0.15 - x

mol of acetate formed = 0 + x

and we know

A-/HA = 1.73780

A-/HA = x /(0.15-x)

1.73780 = x /(0.15-x)

0.15-x = 1/1.73780 x

(1/1.73780 +1)x = 0.15

x = 0.15 / (1/1.73780 +1) = 0.0952

then

M = mol/V

V = mol/M = 0.095 / 3 = 0.03166 L = 0.03166*10^3 mL = 31.66 mL

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