if you add a strong acid (HCl) with a weak (hc2h3o2); how does it effect the % ionization of the weak acid?

Question

so lets say you are given a concentration of hcl = x ; and a concentraction of hc2h3o2 = y
how would you find the the percent ionization and pH of of the acetic acid?

so what formula should i use for calculating % ionization and ph?

Answer 1

You have picked a rather not-so-easy problem to work, but it is workable.

First, you assume all of the strong acid dissociates, and iti s going to dramatically suppress ionization of the acetic acid (it doesn't take much at all ofthe HCL to affect this).

Working from the equilibrium equation:

Ka = [H+][A-]/[HA].

First, let's assign a variable to the amount of acid that ionizes, and let's call this variable z.

Since acetic acid is weak, and especially since we are suppressing what little ionization takes place, [HA] is approximately equal to y, but it is exactly equal to (y-z).

[H+] = x+z; x is most likely going to dominate;
[A-] = z because the Cl- is not a conjugate base worth mentioning (it's the conjugate base of a really strong acid)

now, time for fun with Algebra:

Ka = (x+z)(z)/(y-z) ; x and y are knowns, not variables.

Ka = 10^-pKa = 1.7378 * 10^-5

So... xz+z^2 = (1.7378 *10^-5)*(y-z) or rearranging to get a quadratic equation:

z^2 + (x+1.7378*10^-5)*z - ((1.7378*10^-5) * y) = 0

The positive real root of this equation is your solution, so:

z = [-(x+ 1.7378 * 10^-5) +/- sqrt ((x+1.7378*10^-5)^2 - 4 * (1.7378 *10^-5))]/2

Once you have z, you know [H+] = x+z

pH = -log(x+z)

% ionization = (z/y)* 100 ; it will be SMALL

Have fun plugging in!

Answer 2

If you need to just think qualitatively about this, you can consider leChatlier's principle and the effect of increasing the proton concentration on the right side of the dissociation equation of acetic acid. Adding H+ will shift the equilibrium toward acetic acid, and will decrease the % ionization.

If you have to do this quantitatively, you can use the equation for the equilibrium constant for the dissociation of acetic acid.

The [H+] will be the concentration of the HCl, and [acetic acid] will be the concentration of the weak acid. If you have a value for Ka, you should be able to calculate the concentration of acetate ions in the absence and then in the presence of some amount of HCl.

Hope this helps

Answer 3

Reduces it. The dissociation of the weak acid is represented by a product of the hydrogen ion concentration, and the counterpart ion concentration. If the hydrogen ion concentration is increased by adding a strong acid, the counterpart ion concentration must decrease, by recombination of some of the counterpart ions with hydrogen ions.