Question

You have a 15 mL sample of acetylcholine (a neurotransmitter) with an unknown concentration and a pH of 7.69. You incubate this sample with the enzyme acetylcholinesterase to convert all of the acetylcholine to choline and acetic acid. The acetic acid dissociates to yield acetate and hydrogen ions. At the end of the incubation period, you measure the pH again and find that it has decreased to 6.72. Assuming there was no buffer in the assay mixture, determine the number of nanomoles of acetylcholine in the original 15 mL sample. Remember to use significant figures in your answer.

Answer 1

The change in hydrogen ion concentration from the decrease in pH from 7.69 to 6.72 was used to calculate that there were 5.49 nanomoles of acetylcholine in the original 15 mL sample.

Step-by-step explanation:

To determine the number of nanomoles of acetylcholine in the original 15 mL sample, we must first calculate the change in hydrogen ion concentration due to the pH decrease. The pH decreased from 7.69 to 6.72. The pH is equal to the negative logarithm of the hydrogen ion concentration (-log[H+]). This change represents a 10^(6.72-7.69) or approximately 10^-0.97 change in hydrogen ion concentration.

Calculating the difference gives a concentration change of approximately 10-6.72 - 10-7.69 moles per liter. Since the sample is 15mL (or 0.015L), we multiply this concentration by the volume to find the moles of acetylcholine that were present, since each molecule of acetylcholine would release one hydrogen ion:

(10-6.72 - 10-7.69) * 0.015 L = moles of H+ (and thus acetylcholine) added

moles * 109 = nanomoles of acetylcholine

After performing the calculations:

Number of nanomoles = 5.49 nanomoles of acetylcholine.