Homework Help: Chemistry

Posted by Unknown on Tuesday, February 28, 2012 at 4:28pm.

You are preparing 500 mL of a 0.300 M acetate buffer at pH 4.60 using only sodium acetate, 3.00 M HCl, 3.00 M NaOH, and water. Calculate the quantities needed for each of the following steps in the buffer preparation.

1. Add sodium acetate to ~400 mL of water in a large beaker. How many grams of sodium acetate do you need? Report your answer to the nearest mg.

2. Add 3.00 M while stirring and monitor the pH with a pH electrode until the pH = 4.60. What volume of HCl or NaOH do you calculate that you will need? Ignore activity coefficients and report your answer to the nearest mL.

3. Quantitatively transfer the beaker contents to a 500 mL volumetric flask, fill to the mark with water, and mix thoroughly.

Chemistry - DrBob222, Tuesday, February 28, 2012 at 6:39pm
pH = pKa + log(base)/(acid)
4.60 = 4.74+ log b/a
b/a = about 0.7 but that's approximate and you need to do it better than that.
You want the buffer to be 0.3M in acetate which means
a + b = 0.3
Solve the two equations simultaneously to obtain a and b. I obtained about 0.2M for (a) and about 0.1M for (b).
Since you want only 500 mL of the buffer, divide those numbers by 2 to obtain mol.
For #1, mols b x molar mass NaC2H3O2 = grams sodium acetate.
for #2, M = moles HCl/L HCl. You know moles and M, solve for L and convert to mL.

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