Gen Chem question...

[BrownBradPitt]

---

Members do not see this ad.

I had trouble figuring out why the right answer was right....


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?

A. 2 x 10–3
B. 4 x 10–6
C. 2 x 10–5
D. 4 x 10–10
E. 2 x 10–10

I got B because isnt.002% of 1M = .002 for [A-] and [H+]?

Yet Kaplan's reasoning was this: verbatim

"Here the [HY] = 1 M, and the [H+] = [Y-] = .002% of [HA] = 2x10^-5(1)=2 x 10^-5 M.

Therefore,
Ka = ( [H+][A–] ) / ( [HA] )

Ka = ( (2x10-5)^2) / ( 1 ) = 4 x 10^-10.

Note that choice B, an incorrect choice, is what one would get by forgetting to take into account the fact that 0.002 is a percentage."


Does Kaplan's obscure questions and explanations frustrate anyone else?

 

[thefifthbeatle]

yeah kaplan is tricky....take those subject ones with a grain of salt!!
just remember (this is good for qr, too)

2% - .02
therefore, .002% is .00002

 

[BrownBradPitt]

Thanks! I cant believe how dumb that mistake was.

 

[doc toothache]

yeah kaplan is tricky....take those subject ones with a grain of salt!!
just remember (this is good for qr, too)

2% - .02
therefore, .002% is .00002

There is nothing tricky or obscure about this problem. The amount of ionization is usually expressed as a percentage.