Chemical Forums

Chemistry Forums for Students => High School Chemistry Forum => Topic started by: objectivist on April 03, 2005, 04:19:43 PM

Title: pH of a solution??!!
Post by: objectivist on April 03, 2005, 04:19:43 PM

Hello. Help would be much appreciated!

What is the pH of a solution if 100.0 mL of the solution is 0.21 M with respect to acetic acid and contains 1.59 g of sodium acetate, NaCH3CO2?


i found out the Molarity of sodium acetate to be 0.1939 but from there I am lost. i tried adding the -log of the two concentrations to get the pH but that is wrong.

Can anyone help?

Title: Re:pH of a solution??!!
Post by: objectivist on April 03, 2005, 04:30:42 PM

Do I do an ICE Table?: the

Initial
Change
Equilibrium

thing?

and use "Ka" or something?

Title: Re:pH of a solution??!!
Post by: objectivist on April 03, 2005, 04:40:29 PM

Okay...i think i was looking at the wrong chapter and may have figured it out.

but i am not sure if it is right:

        CH3CO2H + H20  --  H30+   +    CH3CO2-

I          0.21                      0              0.1939

C          -x                        +x                +x

E         0.21 - x                    x             0.1939 + x


Ka  =  (x)(0.1939 + X)  
              0.21 + x

but then cuz "x" is small u take it out....

The Ka of acetic acid is (1.8 e-5) (thats the one i use, right?)

and solve for "x" and get 1.72e-6

-log(1.722e-6)   =   a pH of  5.76 ?!???

Title: Re:pH of a solution??!!
Post by: objectivist on April 03, 2005, 04:43:58 PM

Well...i just found out that that answer is wrong.

Can anyone tell me what I did wrong?

Title: Re:pH of a solution??!!
Post by: pkothari13 on April 03, 2005, 06:09:18 PM

Use the Henderson-Hasselbalch Equation...

pH = pK_a - log([base]/[acid])

The base in this case is CH3CO2- and the acid is HCH3CO2. The pK_a is the same as -log(1.8e-5).

So your final answer is

pH = -log(1.8e-5) - log(.1939/.21)

Tah dah!

Title: Re:pH of a solution??!!
Post by: objectivist on April 03, 2005, 08:10:22 PM

 :D Thank you so much PkoThari13!!!

I dont know what the hell i was doing...