Once the correct ratio of A - to HA is known, determine the actual concentration

Question

Once the correct ratio of A- to HA is known, determine the actual concentration of A- needed to make a buffer at a particular concentration.

The ratio of acetate to acetic acid in a buffer is 0.177. That is, the concentration of acetate should be 0.177 times that of the concentration of acetic acid. What is the concentration of acetate (in M) needed to prepare a 0.403 M buffer using acetic acid and acetate?


What volume of acetate solution (in mL) is needed to make 250.0 mL of a buffer solution? The concentration of acetate in the buffer must be 0.0585 M, and the concentration of the acetate stock solution is 0.500 M.

First, determine the correct A-/HA ratio:  The pKa of acetic acid is 4.76, and the buffer solution needs to maintain a pH of 4.18. The concentration of acetate (CH3CO2-) should be X times that of the concentration of acetic acid (CH3CO2H). What is X?

Explanation / Answer

1) Let us denoted acetate as A- and acetic acid as HA. Therefore, we have,

[A-]/[HA] = 0.177

=====> [A-] = 0.177*[HA] ……(1)

Again, the concentration of acetic + acetic acid in the prepared buffer is 0.403 M. Therefore, we have,

[A-] + [HA] = 0.403 M

====> 0.177*[HA] + [HA] = 0.403 M

====> 1.177*[HA] = 0.403 M

====> [HA] = (0.403 M)/(1.177) = 0.342 M (ans).

Thus, [A-] = 0.177*[HA] = 0.177*(0.342 M) = 0.060 M (ans).

2) Use the dilution law:

C1*V1 = C2*V2

where C1 = 0.500 M is the concentration of stock acetate; C2 = 0.0585 M is the concentration of acetate in the buffer and V2 = 250.0 mL is the volume of the buffer. We are required to find out V1. Plug in values and obtain

(0.500 M)*V1 = (0.0585 M)*(250.0 mL)

====> V1 = (0.0585 M)*(250.0 mL)/(0.500 M) = 29.25 mL.

29.25 mL of stock 0.500 M acetate buffer must be diluted to 250.0 mL to have the concentration of acetate as 0.0585 M (ans).

3) Use the Henderson-Hasslebach equation as

pH = pKa + log [CH3CO2-]/[CH3COOH]

=====> 4.18 = 4.76 + log [CH3CO2-]/[CH3COOH]

=====> log [CH3CO2-]/[CH3COOH] = 4.18 – 4.76 = -0.58

=====> [CH3CO2-]/[CH3COOH] = antilog(-0.58)

=====> [CH3CO2-]/[CH3COOH] = 0.26

=====> [CH3CO2-] = 0.26*[CH3COOH]

Therefore, X = 0.26 (ans).

Related Questions

Navigate

• Browse (All)
• Browse O
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.