Answer
1.79 moles
Work Step by Step
For every acetylcholine, a hydrogen ion is released in the reaction which means [H] produced during the reaction = Acetylcholine concentration.
[H] produced during reaction = [H] at the end- [H] at start of reaction
[H] = $10^{-pH}$
[H] at start of reaction = $10^{-7.65}$ = $0.224 \times 10^{-7}$M
[H] at the end of reaction = $10^{-6.87}$ = $1.35 \times 10^{-7}$M
[H] produced during reaction = $(1.35 -0.224) \times10^{-7}$M =1.13M
This is equal to acetylcholine concentration
$1.13 \times10^{-7}$ moles in 1 liter
In 15 mL or 0.0015 L, the number of moles = $1.13 \times10^{-7} \times 0.0015 L $ = $1.7 \times10^{-9}$ moles
