A buffer may be prepared in a number of ways, three of which are given below: 1.

Question

A buffer may be prepared in a number of ways, three of which are given below: 1. Neutralization of a portion of the weak acid with sufficient strong base to give the desired ratio, [A- ]/[HA]: HA + NaOH Na+ + A- + H2O 2. Neutralization of a portion of the weak base with sufficient strong acid to give the desired ratio, [A- ]/[HA]: Na+ + A- + HCl HA + Na+ + Cl- 3. Combining the weak acid and the conjugate base in amounts necessary to give the desired ratio, [A- ]/[HA]. The three methods will all give solutions with the same pH and buffer ion concentrations, if appropriate experimental techniques are used for buffer preparations. It should be noted, however, that in the case of method 2, the buffer solution will also contain the salt of the strong acid used to neutralize the conjugate base.

1. You are given sodium acetate, 1M HCl, NaHCO3 and Na2CO3. Determine which of these four you would need and then show calculations to make buffer pH = 4.7 by method 2. Assume making 100 mls of a 0.1 M buffer.

2. You are given sodium acetate, 1M HCl, NaHCO3 and Na2CO3. Determine which of these four you would need and then show calculations to make buffer pH = 9.9 by method 3. Assume making 100 mls of a 0.1 M buffer.

3. By consulting a buffer table, suggest a buffer system other than phosphate or carbonate and calculate acid/conjugate base ratio to get a buffer pH = 6.6.

4. After testing the buffer capacity, did the pH values for the solutions you prepared agree with the expected pH based upon your calculations? Explain your results keeping in mind any difference between the obtained pH values and your assigned pH value.

Please help, I am not sure how to do this and I cannot understand my instructor's explanation!

Explanation / Answer

1) You are given sodium acetate, 1M HCl, NaHCO3 and Na2CO3. Determine which of these four you would need and then show calculations to make buffer pH = 4.7 by method 2. Assume making 100 mls of a 0.1 M buffer

solution :

pH = 4.7

pKa of bicarbonate = 6.37

pKA of acetic acid = 4.7

So we will use this acid and its salt

We are provided with salt already

so we need to find out the concentration of acid required

pH = pKa + log [Salt] / [acid]

4.7 = 4.7 + log [salt/[acid]

so concentration of acid and salt should be same

Concentration of sodium acetate = 0.05 = [Acetic acid]

Volume 100 mL

Moles = 0.005 moles of each

We will take 0.05M of 50mL sodium acetate and will react it with 50mL of 0.05 M of HCl

CH3COONa + HCl CH3COOH + NaCl

Then will combine 50mL of above solution with 50mL of 0.05 M of sodium acetate

2) pKa of HCO3- = 10.3

we need to preapre buffer with pH = 9.9

So we will take NaHCO3 and Na2CO3

As we have to prepare 100mL of buffer with concentration 0.1 M

so we have to calculate the portions of the given salt first

pH = pKa + log [Salt] / [acid]

9.9 = 10.1 + log[Na2CO3]/ [ NaHCO3]

[Na2CO3]/ [ NaHCO3] = 0.63

also given [Na2CO3] + [NaHCO3] = 0.1

0.63[NaHCO3] + [NaHCO3] = 0.1

[NaHCO3] = 0.0613 M

Moles = Molaritu X volume = 0.0613 X 100/1000 = 0.00613 moles

[Na2CO3] = 0.1-0.0613 = 0.938 M

Moles = 0.0938 Moles

wo we will preapre the above buffer by adding the calcualted moles in 100mL of solution

Moles = Mass / Molecular weight
3) we can use salt of NaHSO4 and Na2SO4 to get the desired buffer as pKa of NaHSO4= 6.91

Sulphurous buffer

4) The tests are not given

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