The compound shown is stirred with excess NaOD in $\mathrm{D}{}_2\mathrm{O}$ until exchange is complete. Assume that only hydrogens on carbons directly bonded to the carbonyl carbon exchange. How many deuterium atoms are incorporated per molecule?

The compound shown is cyclopentyl methyl ketone. The carbonyl carbon is bonded to the CH${}_3$ group (3 H) and to the ring CH (1 H). These $3+1=4$ alpha hydrogens exchange through the enolate; the ring CH${}_2$ hydrogens are beta and do not exchange, so 4 deuterium atoms are incorporated.