Question

Assume that the reaction used to prepare the buffer goes to completion (i.e., that all the strong base reacts) and use the data in Table A to calculate the initial concentration of the weak acid and its conjugate base in each of the three buffer solutions.

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This experiment is done by students working in groups of three. During Week One, each group will be assigned one specific weak acid to work with: acetic acid (CH3CO2H), formic acid (HCO2H), or propanoic acid (CH3CH₂CO₂H). Each group will prepare three buffer solutions by mixing their assigned weak acid with different amounts of NaOH, as indicated in Table A.

TABLE A

Buffer	mL of 0.50 M weak acid	mL of 0.25 M NaOH	mL of H2O
1	50.00	50.00	0.00
2	50.00	35.00	15.00
3	50.00	25.00	25.00

Answer 1

Buffer 1: The moles of acid and base are calculated:

n acid = M * V = 0.5 M * 0.05 L = 0.025 mol

n base = 0.25 * 0.05 = 0.0125 mol

The remaining moles of acid and the concentrations are calculated:

n acid remaining = 0.025 - 0.0125 = 0.0125 mol

[Acid] = n / v = 0.0125 / 0.1 = 0.125 M

[Base] = 0.0125 / 0.1 = 0.125 M

Buffer 2: The same procedure is used above, and the results are:

n acid = 0.025 mol

n base = 0.00875 mol

n acid remaining = 0.01625 mol

[Acid] = 0.1625 M

[Base] = 0.0875 M

Buffer 3: The results are:

n acid = 0.025 mol

n base = 0.00625 mol

n remaining acid = 0.01875 mol

[Acid] = 0.1875 M

[Base] = 0.0625 M

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