2,2-Dimethoxypropane is hydrolysed in aqueous acid made with $\mathrm{H}{}_2{}_{}^{}{}_{}^{18}\mathrm{O}$ (water enriched in oxygen-18). Where does the ${}_{}^{}{}_{}^{18}\mathrm{O}$ end up?

Hydrolysis proceeds by protonation of an OCH3 group and loss of $\mathrm{CH}{}_3\mathrm{OH}$ (C-O cleavage at the acetal carbon, with the original oxygen leaving as methanol). The oxocarbenium ion is attacked by $\mathrm{H}{}_2{}_{}^{}{}_{}^{18}\mathrm{O}$, and after loss of the second methanol the carbonyl oxygen comes from water. So the label is in acetone and not in methanol.