Butan-2-one is stirred for a long time with excess $\mathrm{NaOD}$ in $\mathrm{D}{}_2\mathrm{O}$ so that every alpha hydrogen exchanges. How many hydrogen atoms of one molecule are replaced by deuterium?

Butan-2-one is $\mathrm{CH}{}_3-\mathrm{CO}-\mathrm{CH}{}_2-\mathrm{CH}{}_3$. The alpha carbons are the methyl on one side (3 H) and the methylene on the other (2 H), so $3+2=5$. The terminal methyl of the ethyl group is beta, so its three hydrogens do not exchange. The total of 8 H therefore includes 3 that stay as H.