Calculating H3O+ Concentration for Acetic Acid Solution | Ka = 1.8 x 10-5

[2200andbeyond]

Homework Statement

What is the H3O+ concentration of a 0.100 M acetic acid solution (Ka = 1.8 x 10-5)?
a. 1.8 x 10-5
b. 1.8 x 10-4
c. 1.3 x 10-2
d. 1.3 x 10-3
e. 0.9 x 10-3

Homework Equations

The Attempt at a Solution

so the question is supposed to be D. but I thought that The Ka tells the concentration of the H3O+! Doesn't it ?!

I think you're supposed to take the square root of 1.8 x 10-5 the multiply it by 0.1
BUT WHY?!

thanks in advance!

 

[Borek]

Not that simple.

http://www.chembuddy.com/?left=pH-calculation&right=pH-weak-acid-base

Alternatively, google for ICE tables. Or see if http://www.chembuddy.com/?left=BATE&right=pH-cheat-sheet doesn't help - just note it doesn't explain/teach anything.

 

[epenguin]

I think you're supposed to take the square root of 1.8 x 10-5 the multiply it by 0.1
BUT WHY?!

Something that might sound like that but you have got it jumbled.

The answer and the right equation is explicitly there in the source given by Borek but I am not sure it would be helping you to direct you to it. You will forget the formula again like you already have by the next time you need to use it, so you need the ability to work out what it is.

Write down the equations you have to use, and if you do not then see, show them to us and we will further help.