I hate people like that.. (unfortunately 98% of my school..)
The best way to deal with this is to kind of not TELL her to do anything, but you have to sort of act really annoyed.
Ex.
Your friend: " O_O' TAN Whats 8+1 again? "
In your response, you have to act annoyed, and sort of hint that she's an idiot (it's mean, but necessary)
You: "*Annoyed face, demeaning tone.* Lets see, add the 12, carry the five, divide by 0.. oh yea, 8+1 is 9"
Your in a tricky situation, I imagine you still want to be friend with her, so telling her is going to be pretty hard.
If the above approach doesn't work, just try ignoring the really basic and silly questions.
If she's asking for the molar mass of magnesium, just tell her how to obtain the answer (i.e "check the periodic table")