．、。
5. Once the reaction is complete, place the test tube on a wet rag.
Literal
5. Reaction [subject-が] end-[when-たら], wet rag [genitive-の] top [on-に] test-tube [object-を] place.
Step 5 of a chemistry lab procedure. Note the use of plain verb-ending おく (a dictionary form) rather than polite 置きます — this is the instructional-manual register, which uses plain-form verbs throughout for terseness and neutrality, even in otherwise formal documents. The ~たら conditional here is the 'once X happens' temporal use, not hypothetical: 'once the reaction has finished, [do Y].' ~の上に ('on top of X') is the standard construction for placing something atop another — the noun 上 ('top') plus の and に.