G Chem Problem

[DMD to Be]

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The Ka of acetic acid is approximately 2x10^-5. What is the pH of a 0.5 M solution of acetic acid?

A. 1.0
B. 2.0
C. 2.5
D. 3.0
E. 7.0

The answer is C.

This problem is confusing to me, and was wondering if anyone has seen anything this complicated on their DAT. There were 3 or 4 very simliar questions to this on the kaplan practice tests and they are all complicated with the -logs and all.

 

[sgdorfl90]

for this, you would need to set up quadratic function. Ka of acetic acid shows its not so strong to have full deprotonation. So what we need to do is set up an ice box. we have .5M of acetic ion-H <~~~...i am doing this to show the H. Now, when we deprotonate X amount, it will decrease from .5, so .5-x( this is our numerator). On the product side, we have now X of H+ and X of acetate ion in equal conc. So it would now be X^2/(.5-X)=Ka of acetic acid. Solve for x, that's the M of H+. -log(X)= pH

 

[lansingMN]

Assume that this is a strong acid, it dissociate completely.

HCl -----> H+ + Cl-

Ka= [H+][Cl-]/HCl

in another word it's 2 x 10^-5= x^2/.5

x^2 = (2x10^-5)(.5)

x^2 = (1 x 10^-5)

x= sqrt (1 X 10^-5)

if you write this out... ( 1 x 10^-5)^(1/2) on paper u'll see it's (-5*1)/2 is -2.5 and 1 to any power is 1... so concentration of H+=Cl-=(1X10^-2.5)... so pH= 2.5

I hope you understand what i mean... sorry if it confuses u.

 

[lansingMN]

sgdorfl90,

I thought u need not to set up quaratic equation unless the x u found is lower than 5%. Also, this Ka is quite small.

 

[sgdorfl90]

sgdorfl90,

I thought u need not to set up quaratic equation unless the x u found is lower than 5%. Also, this Ka is quite small.

ya, you are right...its a safe way to get exact answer. on the real exam, I guarantee everyone you will not see something this difficult, but know the concept behing why we do what we do. Know chemistry, that's all. Don't go crazy over math. Highly recommended book is Examkrackers 1001 G-chem.

 

[Djapprentice]

ya, you are right...its a safe way to get exact answer. on the real exam, I guarantee everyone you will not see something this difficult, but know the concept behing why we do what we do. Know chemistry, that's all. Don't go crazy over math. Highly recommended book is Examkrackers 1001 G-chem.

heh