Hey, anyone know how I would work this out?

Calculate the pH of 0.1M dichloroacetic acid, Ka = 3.3 x 10-2.

Thanks :D

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- Feb 5th 2008, 05:55 AMDan167pKa and pKb calculation
Hey, anyone know how I would work this out?

Calculate the pH of 0.1M dichloroacetic acid, Ka = 3.3 x 10-2.

Thanks :D - Feb 5th 2008, 11:34 AMJhevon
we are given little to work with here, we have to go the long way around. (unless you are dealing with a buffer and you neglected to mention the molarity of the basic part of the acid). what is the formula for dichloroacetic acid? we have to write an equation for the ionization reaction using it

- Feb 5th 2008, 11:43 AMearboth
Hi,

have a look here: Dichloroacetic acid - Wikipedia, the free encyclopedia - Feb 5th 2008, 12:26 PMbobak
well we have enough information to solve his (providing we make a few assumptions)

$\displaystyle Ka = \frac{[H+] \cdot [salt]}{[Acid]}$

if we assume that the solution is not buffered then $\displaystyle [H+] = [Salt] = x$

so then $\displaystyle Ka = \frac{x^2}{[Acid] - x } $

and there you have enough information to solve the equation. - Feb 5th 2008, 12:31 PMJhevon
- Feb 5th 2008, 12:39 PMbobak
well in chemistry class we are usually allowed to assume that the change in concentration of the acid may be neglected as it is so small so most people just leave the denominator as [Acid]. however this assumption is only really valid for very small values in Ka, but chemistry teachers don't really want to waste time teaching students how to solve quadratic equations so they usually make the assumption to allow easier computation of the ph

- Feb 5th 2008, 01:56 PMDan167
Wow i'm confused :o ... Thats all that is given I guess we have to make assumptions.... Does anyone mind going through it step by step :D

- Feb 5th 2008, 02:02 PMJhevon
- Feb 5th 2008, 02:07 PMDan167
- Feb 5th 2008, 02:10 PMJhevon
you have $\displaystyle K_a = \frac {x^2}{0.1 - x}$

$\displaystyle \Rightarrow K_a(0.1 - x) = x^2$

$\displaystyle \Rightarrow x^2 - K_a(0.1 - x) = 0$

$\displaystyle \Rightarrow x^2 + K_ax - 0.1K_a = 0$

plug in the value for $\displaystyle K_a$, you will get a quadratic equation. then just use the quadratic formula to solve for $\displaystyle x$ - Feb 5th 2008, 02:16 PMDan167
hate to ask this but whats the quadratic formula (Blush)

- Feb 5th 2008, 02:25 PMJhevon
- Feb 5th 2008, 02:27 PMDan167
- Feb 5th 2008, 02:38 PMDan167
Ok nope i'm lost .... :rolleyes:

- Feb 5th 2008, 02:40 PMJhevon