Determining Charge of Protonated Alkaloids
Are you sure you're trying to calculate the molar mass of the mescaline salt? Isn't that known, depending on the acid you use? 

It seems to me the key you need is the mole ratio. For a weak acid-base conjugate pair, you can use the Henderson-Hasselbalch equation:

$pH = pK_a + log(\frac{[B]}{[A]})$

where [B]/[A] is the ratio of concentrations of conjugate base to conjugate acid. If calculated for a single solution containing both forms, the total solution volume cancels and this is the mole ratio. In a more convenient form,

$\frac{B}{A} = 10^{pH - pK_a} = \frac{K_a}{[H^+]}$

(The first equality is more practical, the second is useful for theoretical manipulations.)

Note that when $pH = pK_a$, $B = A$ (the analyte will be 50% protonated and 50% deprotonated).

For mescaline (more precisely, its conjugate acid), $pK_a =$ 9.56, typical of an amine base/alkaloid. So at a pH of 9.56, approximately half of your mescaline should be protonated. You could extract this, measure the weight of the salt, use the known molar mass to convert to moles of the protonated mescaline (I recommend using a monoprotic acid such as acetic if possible to simplify the situation). By the Henderson-Hasselbalch, the moles of protonated mescaline you extracted at its $pK_a$ will equal the moles of freebase mescaline remaining in your organic solvent.