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_a=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? 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.) 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? 1 2 20

Explanation / Answer

First, determine the ratio of A- /HA needed for pH 5.00 by rearranging the Henderson Hasslebach equation:

mol A-/molHA = 10pH-pKa =  105-4.76 = 1.74 mol
mol A- = 1.74 mol HA

We also know that the total moles of HA and A- = 0.500 L x 0.300M = 0.1500 mol.

So: mol A- + mol HA = 0.1500
and mol A- = 1.74 mol HA

Solving for each term, we find:
0.15 = 2.74mol HA
mol HA = 0.0547 moles
mol A- = 1.74(0.0547) = 0.0951 moles

Quantities for each:
We need a total of 0.1500 moles of acetic acid:
m = 0.15 mol * 60.05 g/mol HA = 9.01 g acetic acid

We need to convert 0.0951 moles to acetate by adding NaOH so:
Vb = 0.0951 moles * 1 L/3moles =  0.0317 L or 31.7 mL

Part c I think is 20. Hope this helps

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