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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: moni on April 10, 2006, 12:27:21 PM

Title: % ionization,pH, concentration
Post by: moni on April 10, 2006, 12:27:21 PM

a) the ionization constant Ka of formic acid ,HCO2H is 1.80 *10^-4. What is the percent ionization of a 0.00100 M solution of this acid?

b)The  pH of a 0.072 M solution of benzoic acid HC7H5O2 is 2.86.compute  Ka for this acid?

c)What concentration of acetic acid HCH3CO2 is needed to give a [H+]=3.5 *10^-4? Ka = 1.75 *10^-5

For the a) i'm not sure  but :

HCO2H ----> HCO2-  + H+

% ionization =HCO2H/H+  *100%

Ka = [HCO2-][H+]/[HCO2H]
1.8*10^-4 = (x)(x)/0.417M
x^2=(1.8*10^-4)(0.417)
x=8.6*10^-3

% ionization= 1.80*10 ^-4/8.6*10^-3 *100%
% ionization = 2.09%

am i on the right truck?

Title: Re: % ionization,pH, concentration
Post by: Borek on April 10, 2006, 12:41:09 PM

Quote from: moni on April 10, 2006, 12:27:21 PM

1.8*10^-4 = (x)(x)/0.417M

What is 0.417?

Regardles of where did you get this number from - the equation is wrong. Numerator depends on x.

Quote

am i on the right truck?

Depends on where you are going :)

Title: Re: % ionization,pH, concentration
Post by: moni on April 10, 2006, 02:08:41 PM

Numerator depends on x.

what does it mean?

Title: Re: % ionization,pH, concentration
Post by: Borek on April 10, 2006, 02:17:43 PM

Sorry - I meant denominator is a function of x (numerator is function of x too, but that is already visible in your formula).

Title: Re: % ionization,pH, concentration
Post by: moni on April 10, 2006, 02:52:32 PM

but like u said the equation is wrong,that means  everything is wrong

Title: Re: % ionization,pH, concentration
Post by: Borek on April 10, 2006, 02:55:56 PM

You are - unfortunately - right.

http://www.chembuddy.com/?left=pH-calculation&right=pH-weak-acid-base

http://www.chembuddy.com/?left=BATE&right=pH-cheat-sheet

Title: Re: % ionization,pH, concentration
Post by: moni on April 10, 2006, 04:23:20 PM

HCO2H + H2O----> HCO2- + H3O  is it right? 

Title: Re: % ionization,pH, concentration
Post by: Borek on April 10, 2006, 04:25:53 PM

Yes, but you may safely omit water for these calculations, and assume it is H⁺, not H₃O⁺. There is less writing when solving ;)

Title: Re: % ionization,pH, concentration
Post by: moni on April 10, 2006, 04:32:55 PM

HCO2H ----> HCO2-  + H+ so that is good too

Ka is wrong, but from  Ka = [H][A] /[HA]

Ka = [HCO2-][H+] / [HCO2H]

Title: Re: % ionization,pH, concentration
Post by: Borek on April 10, 2006, 04:46:05 PM

So far so good.

Title: Re: % ionization,pH, concentration
Post by: moni on April 10, 2006, 05:17:20 PM

1.8 *10^-4 =(x)(x) /x-0.0100

Title: Re: % ionization,pH, concentration
Post by: Borek on April 10, 2006, 05:27:25 PM

0.01 or 0.001?

And x-0.01 or 0.01-x?

Title: Re: % ionization,pH, concentration
Post by: moni on April 10, 2006, 08:43:59 PM

0.01-x

Title: Re: % ionization,pH, concentration
Post by: Borek on April 11, 2006, 02:24:36 AM

So far so good. What next?