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### Topic: Finding initial pH of titration solution  (Read 23729 times)

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#### Defiledxhalo

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##### Finding initial pH of titration solution
« on: August 08, 2012, 01:39:06 AM »
Below is the question:
Consider the titration of a 20.0 mL sample of 0.105 M HC2H3O2 with 0.125 M NaOH. Determine the initial pH levels.
So I know the pH = -log[acid], so I assumed the way to go about this problem is just pH = -log(0.105), which gives me 0.979, which is wrong. The book says the answer is 2.86.
Could someone help explain how I would set up this problem? Thanks!

#### sjb

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##### Re: Finding initial pH of titration solution
« Reply #1 on: August 08, 2012, 02:39:17 AM »
Below is the question:
Consider the titration of a 20.0 mL sample of 0.105 M HC2H3O2 with 0.125 M NaOH. Determine the initial pH levels.
So I know the pH = -log[acid], so I assumed the way to go about this problem is just pH = -log(0.105), which gives me 0.979, which is wrong. The book says the answer is 2.86.
Could someone help explain how I would set up this problem? Thanks!

Is C2H3O2H a strong acid?

#### mugabo daniel

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##### Re: Finding initial pH of titration solution
« Reply #2 on: August 09, 2012, 05:30:31 PM »
pH = -log[acid], this is not correct rather pH = -log[H+]
in this case [acid] = [H+] since bascity of the acid is 1
but 20 ml of 0.105 M contain 0.0021 moles of H+
pH = - log[0.0021]
= 2.68
this is what i get, please guys correct me if am wrong.

#### grs35

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##### Re: Finding initial pH of titration solution
« Reply #3 on: August 13, 2012, 12:35:05 AM »
ok first thing you need to know is that acetic acid is a weak acid and you need to write down the acid dissociation reaction.

acetic acid+h20 --> h30+ +c2h3o2

i dont know how much you know about titration, but you have to set up an ICE table. the initial concentration of acetic acid is 0.105 M and the rest is zero.

so, you write the equilibrium expression as x^2=1.8.10^-5X(0.105-x)

since the reaction goes against the overall reaction, you can use small x approximation. if you solve for X you will find 1.375X10^-3 M this is the concentration of H30+

negative log of it will give you the pH which is 2.86169...

#### AWK

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##### Re: Finding initial pH of titration solution
« Reply #4 on: August 13, 2012, 05:27:51 AM »
In this approximation showing more than 2 significant digits is a serious error (since used Ka=1.8.10-5).
Note - in logaritms significant digits are after decimal point.
AWK

#### mugabo daniel

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##### Re: Finding initial pH of titration solution
« Reply #5 on: August 13, 2012, 05:57:11 AM »
thanks for that approach guys. guess i got confused somewhere.