[roundeggy]

A particular sample of vinegar has a pH of 2.90 assuming that acetic acid is the only acid that vinegar contains (Ka equals 1.8X 10-4) calculate the concentration of acetic acid in the vinegar...
 so i did 10^(-2.90) which gave me (1.26X10^-3) then i squared that and divided it by (1.26X10^-3-X) = Ka (1.8X10^-4) ... i figured i should multiple the Ka by the lower half of the fraction but for some reason i keep getting the value of X wrong. Please write back what you did clearly thanks!

[1.26X10^-3]^2/[1.26X10^-3-X]= 1.8X10^-4 wat is X

[Train]

Why did you write (1.26*10^-3 - x)?

[roundeggy]

i wrote that for the bottom of the fraction to figure out what the concentration of {H+}... what would u have done?

[Train]

You already know [H+] - from the pH.  As you wrote: [H+] = 10^(-2.90) = 1.26X10^-3

Why are you subtracting x from that number in the denominator?

[roundeggy]

So you would just multiple Ka time the concentration then square root that answer?

[Train]

No. 

You should write the expression for Ka.  After you have an expression for Ka, start trying to relate any unknowns until you only have 1 unknown left. 

How to find the relation you're looking for: Since acetic acid is a weak acid, write your equations as if it is initially undissociated and then partially dissociates into the conjugate base and H+.  You don't know how much acetic acid was initially present but if you assume all of the H+ is from acetic acid then you do know how much of it dissociated.  Your unknown, x, is the amount of acetic acid before it dissociated.

[AWK]

A particular sample of vinegar has a pH of 2.90 assuming that acetic acid is the only acid that vinegar contains (Ka equals 1.8X 10-4) calculate the concentration of acetic acid in the vinegar...
 so i did 10^(-2.90) which gave me (1.26X10^-3) then i squared that and divided it by (1.26X10^-3-X) = Ka (1.8X10^-4) ... i figured i should multiple the Ka by the lower half of the fraction but for some reason i keep getting the value of X wrong. Please write back what you did clearly thanks!

[1.26X10^-3]^2/[1.26X10^-3-X]= 1.8X10^-4 wat is X

Ka equals 1.8X 10-5

[Borek]

i wrote that for the bottom of the fraction to figure out what the concentration of {H+}... what would u have done?

You are trying to use ICE table not knowing what I, C and E stand for.

http://en.wikipedia.org/wiki/ICE_table

[jpg28]

I agree with AWK, the K_a for acetic acid is 1.8 x 10^-5. Maybe that's the reason why the wrong value for x keeps on appearing. Try replacing the K_a with this one, and maybe the correct value for x might show up. This is just to reinforce AWK's post, by the way. Have a nice day and hope you'll be able to solve that.