Question...

exam #3 from winter 2006

Question #2 - could you explain how to determine if it is an effective buffer for each combination listed

Thank you

2. Does the combination listed result in an effective buffer solution? (4pts each)

Yes No 0.38mol HCl(aq) + 0.38mol NaOH(aq)

Yes No 0.90mol Na₃PO₄(aq) + 1.35mol HNO₃(aq)

Yes No 1.28mol Na₂CO₃(aq) + 0.64mol HCl(aq)

Yes No 2.14mol CH₃COOH(aq) + 1.96mol CH₃COOK(aq)

Yes No 0.06mol HCN(aq) + 0.98mol LiCN(aq)

An effective buffer contains approximately equal amounts of a weak acid and its conjugate base with the concentration of each at least 100x the K_a of the conjugate acid. So let's look at each combination:

0.38mol HCl(aq) + 0.38mol NaOH(aq)

This is a strong acid and a strong base, the result of this combination will give a solution of sodium chloride in water. Not a buffer.

0.90mol Na₃PO₄(aq) + 1.35mol HNO₃(aq)

Nitric acid is a strong acid, so when 1.35mol of strong acid are added to the phosphate (a weak base), the result of this combination will be 0.45mols of HPO₄^2-(aq) and H₂PO₄^-(aq). This is a combination of a weak conjugate acid and its weak conjugate base. This will be a good buffer.

1.28mol Na₂CO₃(aq) + 0.64mol HCl(aq)

Similar to the previous example, the result of mixing these solutions together will be a combination of 0.64mols HCO₃^-(aq) and CO₃^2-(aq). This will be a good buffer.

2.14mol CH₃COOH(aq) + 1.96mol CH₃COOK(aq)

Acetate ion is the conjugate base of acetic acid (weak acid), and this is an approximately equimolar mixture of acetic acid and acetate ion. This will be a good buffer.

0.06mol HCN(aq) + 0.98mol LiCN(aq)

HCN is a weak acid and cyanide ion is its conjugate base, but the concentrations are too far apart to make an effective buffer. The concentration of the conjugate acid and conjugate base should be within a factor of 10 to make as effective buffer. This is not an effective buffer