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# Calculation: Acetate Buffer pH

BIOCHM-M39EDP

A student was asked to make up a buffer for an enzyme assay in which the optimum pH of the enzyme is 5.2.

The student found the following instructions for an acetate buffer containing 46.3 mL of a 0.2 M solution of acetic acid and 37.0 mL of a 0.2 M solution of sodium acetate in a total volume of 100 mL. The ${ pK }_{ a }$ for acetic acid = 4.76.

The student made up the buffer, but instead of adding 37.0 mL of sodium acetate, they added 3.70 mL and then brought the final volume to 100 mL as instructed.

Which of the following CORRECTLY describes the consequences of the student’s mistake in preparing the buffer?

A

Although the student did not add enough base, the solution will still buffer within ${ \pm 1 }$ unit of the ${ pK }_{ a }$ value of 4.76 and therefore be suitable for the enzyme assay.

B

Since the optimum pH for the enzyme is 5.2, an acetate buffer with a ${ pK }_{ a }$ value of 4.76 would never be a suitable buffer for this enzyme.

C

The original instructions for the buffer were not correct for making a solution with a pH near 5.2. The pH of the solution increased from 3.60 to 4.67 with the student’s error in measuring, making it a better buffer for the enzyme assay.

D

The pH of the buffer decreased from 4.66 to 3.66 with the student’s error. It would not be an effective buffer for an enzyme with an optimal pH = 5.2.

E

The pH of the buffer increased from 4.86 to 5.86 with the student’s error. It would still be an effective buffer for an enzyme with an optimal pH = 5.2.