Solution
= Solution
For the stated single elementary step and constant reaction volume,
$$
v=-\frac{d[\mathrm{MeX}]}{dt}
=-\frac{d[\mathrm{OH^-}]}{dt}
=\frac{d[\mathrm{MeOH}]}{dt}
=\frac{d[\mathrm{X^-}]}{dt}
=\boxed{k[\mathrm{MeX}][\mathrm{OH^-}]}.
$$
The equal magnitudes follow from the unit stoichiometric coefficients. In the buffered case this <hydroxide> consumption is balanced by buffer chemistry; the measured total <hydroxide> <concentration> can then remain constant.