Can you guys me and do a grammer check on this paragraph? and tell me anything else i might have left out?

the events that led to the hungergames was when the 12 districts rebelled against the capital. the purpose of the games were to remind the districts that they were at there mercy. i think that the events that led to the hungergames was the 12 districts rebelled because stated in the text, "then came the dark days, the uprising of the districts against the capital, twelve were defeated the thirteenth obliterated." i picked this quote because it shows that the districts were treated poorly(or were greedy) so they rebelled against the capital which led to the hunger games. the purpose of the hungergames were meant to serve is to remind the districts who was in charge ."taking the kids from our districts, forcing them to kill each other while we watch this is the capitals way of saying how totally we are at there mercy." enforces my fact.

I fixed all your mistakes here is the new document
The events that led to the hunger games was when the 12 districts rebelled against the capital. The purpose of the games was to remind the districts that they were at their mercy. I think that the events that led to the hunger games were the 12 districts rebelled because stated in the text, "Then came the dark days, the uprising of the districts against the capital, twelve were defeated the thirteenth obliterated." I picked this quote because it shows that the districts were treated poorly(or were greedy) so they rebelled against the capital which led to the hunger games. The purpose of the hunger games was meant to serve is to remind the districts who was in charge ."Taking the kids from our districts, forcing them to kill each other while we watch this is the capital's way of saying how totally we are at their mercy." Enforces my fact.

Do you want a grammar check or a writing check? 

Sentence 1I believe “hunger games” should be two words. And is it capped in the book? (I don’t remember offhand.) And “capital” should be “Capitol.” Capped and spelled with an “o.” 

Sentence 2The purpose of the games *was* . . . — “purpose” is the subject of the sentence and it is singular. 

Sentence 2. . . they were at /their/ mercy — you’re referring to people here, so you need the pronoun.
But there’s a problem here also with pronoun reference. Who does the “their” refer to? It refers back to “the capital,” right? In the meantime, there’s another noun in this sentence (“districts”) that “their” will seem instead to refer to. 
You should revise.

Sentence 3Your quote is a little off in spelling, caps, and punctuation. 
Here’s the actual quote — 
Then came the Dark Days, the uprising of the districts against the Capitol. Twelve were defeated, the thirteenth obliterated.

Sentence 4 
Add a space in front of the parens. Put a comma in front of “so” — because “so” is a coordinating conjunction connecting two full clauses. Let me know if you don’t know what that means. 
Correct “capital.” 
Comma between “capital” and “which” because “which” is a relative pronoun and the clause is nonrestrictive, which just means that it restates something about the Capitol that you could also leave off WITHOUT changing the reader’s understanding about which Capitol you meant. (There is only one. There aren’t two.)
Let me know if you don’t understand this. 

Sentence 5
Fix “hungergames.” (It’s not one word, for sure.)
The sentence is a bit skewed. This framework — “The purpose . . . . [was] meant to serve is to remind” — doesn’t work well. That’s likely because this is you thinking through your answer and thinking about what you want to say. You got a bit tangled up. Now say that sentence out loud. This is probably not how you actually speak. 
You might try this — “The purpose of the games was to remind the districts who was in charge.” 
Or this — The games served to remind the district who was in charge.” 
But you’re going to have to go back to rethink those opening sentences as well. 

Sentence 6
Your quote has some errors. You can find out what these are by searching online. 

Sentence 7
Grammatically correct, but not very effective. 

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This is the formula ph + poh = 14 11.8 + poh = 14poh = 2.6then to find concentration, the formula ispoh = -log[oh^-]2.6 = -log[oh^-][oh^-] = 2.5119×10^3 m