Quick Acid Question

[BiomajorPreDent]

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An unknown weak monoprotic acid, HY, is found to be 0.002% ionized in a 1.0 M aqueous solution. What is the Ka of HY?

I know this is a simple one, but I dont even understand the explanation in the answer key. Can anyone solve this really fast before I post the explanation? I think its a typo but im prolly wrong.

 

[pbure9]

I just solved this really quickly.. not sure if its right.. can you post the right answer and I'll post the explanation. Is Ka = 2x10^-5?

 

[BiomajorPreDent]

The explanation says it is 4 * 10^-10

It says [HY] = 1 M, and the [H+] = [Y-] = .002% of [HA] = 2* 10^-5(1)= 2*10^-5 M.

Therefore Ka = [H+][A-]/[HA] = (2*10^-5)^2 / 1 = 4 * 10^-10


I dont get what its saying...lol

 

[pbure9]

figures i did it way too quickly. Forgot to sqaure it basically.

Here's my explanation.
First draw the equation:

HA --> H+ + A-

HA is your acid and it dissociates into H+ and A- (your salt or base)

If it ionizes .002% you take that percentage from 1.0M (concentration of the acid). They're giving you the concentration of H+ and A- here. So .002% of 1 is 2x10^-5.

Set your Ka equation: [H+][A-]/[HA] = (2x10^-5)(2x10^-5)/1
=
4x10^-10

Hope this helps

 

[BiomajorPreDent]

omg, thank you so much

I was being ******ed and making it .002 instead of making it a "decimal" and making it .00002

thanks

 

[miedvied]

figures i did it way too quickly. Forgot to sqaure it basically.

Here's my explanation.
First draw the equation:

HA --> H+ + A-

HA is your acid and it dissociates into H+ and A- (your salt or base)

If it ionizes .002% you take that percentage from 1.0M (concentration of the acid). They're giving you the concentration of H+ and A- here. So .002% of 1 is 2x10^-5.

Set your Ka equation: [H+][A-]/[HA] = (2x10^-5)(2x10^-5)/1
=
4x10^-10

Hope this helps

You know, I knew exactly how to do that, and had a complete brain-fart when it actually came to doing it. Just goes to show the benefit of practice 😉