Where is the treatment plan for the "Citadel: Chemical Treatment" mission?

The chemicals can be found in the mission N7: Fuel Reactor. Head up to the left area at the start, on a datapad that overlooks the radiation below you. It can also be purchased from the Spectre Requisitions Office if you did not find it during the mission. Once acquired, return to the hospital and speak to the salarian doctor, whose name is Dr Silon, to complete the mission.

Completion gives you 5 reputation, 30 experience and 1000 credits. It also updates the Alliance First Fleet War Asset.

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This is found on the mission "N7: Fuel Reactors" which you'll first have access to around the time of the Priority: Rannoch mission.

The item is located between the start of this mission and the place where the Captain is standing. It's sitting on top of some barrels.

Most of these items are missable, but they can then be purchased at the Citadel afterwards for a nominal fee. If you've already completed this mission, check the shops at the Citadel.

The Chemical treatment plan can be acquired while you're taking on the N7 quest called named Fuel Reactors.

Once you start the missing, head towards the end of the area. To the right you should see a crate, and on it, a Medical Treatment Plan. This item allows you to complete the Chemical Treatment quest.