Solving for Concentration of Ethanoate Ions in Ethanoic Acid Solution

[dav1d]

Homework Statement

What concentration of ethanoate (acetate) ions is
expected in a 0.18 mol/L solution of ethanoic
(acetic) acid? Ka is 1.76 x 10-5 at 25°C.

Homework Equations

Ka=products/reactants

The Attempt at a Solution

I know that Ka=1.76 x 10-5=products/reactants but I really don't know what to do from there. I have tried the ICE box method; initial, change, equilibrium but that doesn't seem to work. My textbook only covers the ICE box method and the basic formula, help is appreciated!

 

[Borek]

Show your take with the ICE method, not only it is perfectly suitable here but it also works exactly as expected.

 

[dav1d]

hydrogen acetate acetic acid
initial: 0 0 0.18mol/L
change: - +x -x
equilibrium: 0 0+x 0.18-x

1.76x10^-5=(0.18-x)/x
x=.1799999 mol/L

but apparently the answer is 1.8x10^-3 mol/L.

Am I missing something?

 

[Borek]

hydrogen acetate acetic acid
initial: 0 0 0.18mol/L
change: - +x -x
equilibrium: 0 0+x 0.18-x

1.76x10^-5=(0.18-x)/x

Am I missing something?

Yes, your ICE table is incomplete (what happens to H⁺ concentration during acid dissociation?) and your Ka expression is umop apisdn.

To some extent it doesn't matter, but the convention is to write dissociation reaction starting with undissociated substance as reactant, and to make ICE table for such a reaction. You reversed everything.

 

[dav1d]

so would it be

hydrogen acetate acetic acid
initial: 0 0 0.18mol/L
change: +x +x -2x
equilibrium: x x 0.18-x

1.76x10^-5=(o.18-x)/x^2
that still gives an incorrect 0.18 or -1.76...

What do you mean the Ka is upside down? Isn't it products/reactants?
So how should I write the table using the 'normal' convention?

 

[Borek]

What are products of the dissociation reaction?

 

[dav1d]

What are products of the dissociation reaction?

hydrogen and acetate are dissociated from the acetic acid.

 

[Borek]

Isn't it products/reactants?

hydrogen and acetate are dissociated from the acetic acid.

So if hydronium (not hydrogen!) and acetate are products, why do you put them in denominator?

 

[dav1d]

So if hydronium (not hydrogen!) and acetate are products, why do you put them in denominator?

lol I guess I should have written the equation first. will update soon.