Hey, anyone know how I would work this out?
Calculate the pH of 0.1M dichloroacetic acid, Ka = 3.3 x 10-2.
Thanks
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
Hi,
have a look here: Dichloroacetic acid - Wikipedia, the free encyclopedia
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.
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
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 $\displaystyle x$