Give the mechanism of esterification of a general carboxylic acid RCOOH with methanol in which the alcohol oxygen is labeled with the ${ }^{18} \mathrm{O}$ isotope $\left(\mathrm{CH}_{3}{ }^{\mathrm{i} 8} \mathrm{OH}\right)$. Does the labeled oxygen appear in the ester or in the water product?