Ka, Kb, calculating concentration of F-

[ronpaulkid]

Homework Statement

Calculating concentration of F- for .158M of HF?
Calculate the equilibrium concentration of F- for .158M of HF in the reaction below.?
HF+H20=H30+F-

The Kb is 6.66*10-4

Homework Equations

Kw=Ka*Kb
Ka=[H+][A-]/[HA]

The Attempt at a Solution

I figured that you would find the Ka by taking 1*10^-14 over the Kb.
Since Ka=[H+][A-]/[HA], then Ka=x^2/.158M

However, the answer is not 1.54E-6.

I also made an ICE chart and solving for x using the quadratic formula. I must be missing something. Thanks!

 

[ronpaulkid]

hey, I am new here. did i do something wrong?

 

[flyingpig]

Your equation is ambigious, there is no equal sign in a chemical reaction.

If I remember, HF is a strong acid, it only proceeds in one direction.

 

[Borek]

hey, I am new here. did i do something wrong?

Yes - this is not physics, this is chemistry, and chemistry - when it comes to homework - classifies as other sciences...

Your equation is ambigious, there is no equal sign in a chemical reaction.

This is a perfect shortcut, especially for an equilibrium reaction.

If I remember, HF is a strong acid, it only proceeds in one direction.

HF is a weak acid. Besides, even strong acids are never 100% dissociated (although in most cases that's a perfect approximation).

Since Ka=[H+][A-]/[HA], then Ka=x^2/.158M

0.158M is a formal concentration of acid, not equilibrium concentration of HF.

I also made an ICE chart and solving for x using the quadratic formula. I must be missing something. Thanks!

Show your ICE table, it should yield a correct result.

Edit: actually it seems like acid is dissociated only slightly, even first approach should give answer reasonably close to reality. Are you sure 6.66*10^-4 is Kb, and not Ka?

 

[ronpaulkid]

--------HF+H20=F-+H30
I-----.158---0-----0-----0

C-----(-.158)--0---.158---0

E------x-------0------(-x)----0

Ka=x^2/[x-.158]

Kb=[x-.158]/x^2

Is this close to how it looks?

 

[Borek]

Initial is OK, but later you do some strange tricks.

Have you switched C & E lines?

Why do you think ALL HF was dissociated?