Can you get a hextech chest on a S rating champ?

No. You get a chest for receiving S grades when you have avaiable chests to be received, and are not currently considered with negative behaviour (Recent Restrictions/Bans).

That means that once you are ellegible to receive chests again you will also have to get S ranks to obtain the chests. You can't obtain S ranks, and once your punishment expires obtain the chests.

The same is valid for when you're out of chests to be gained. You can only gain another chest when you get one avaiable (You get one new available per week), even if you get S ranks.

No.

Once you get a S rating on a specific champion and get a chest, you cannot receive another chest for that champion until the next season.

http://na.leagueoflegends.com/en/site/2016-season-update/champion-mastery.html

When you or someone in your premade party earns any S rank, you’ll acquire a chest. You can earn up to four chests each month, and each champion you play can earn you one chest per season.

Chests can be purchased in the shop for 125 RP as well.

Also note that if you are playing for example Graves in a premade team. If someone on your team gets an S the chest you receive will count towards Graves making you ineligible to receive another chest while playing Graves, not the champion they played.

You can see which champions you have or haven't earned chests for on your profile page under the champions tab.

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