Achieve Week 2 Question 2

[Abby Citro 2A]

"The Ka of a monoprotic weak acid is 0.00675. What is the percent ionization of a 0.177 M solution of this acid?"

I used an ICE table to determine the formula, and did 0.00675 = x^2/(0.177 - x) to get x = 0.1058. After that, I did 0.1058/0.177 x 100 to get the percent ionization value of 69.77 percent, which is incorrect.

Does anyone know where I went wrong?

[Jayden Arevalo 2B]

Hi Abby, I think you miscalculated your x, because I am getting for my x value = .031354. Also, one thing you should notice is that your percentage of ionization is really high, which is atypical considering that you are starting with a weak acid; since weak acids do not dissociate very much. Thus, your percentage ionization should be around 17.71 percent.

[Mia Glinn 1I]

Hi! You have the steps all done correctly, and it seems like you understand how to set up ice tables and solve for percent ionization based on that. It must just be a algebra error, and then you should be on your way to getting the correct answer. Remember for this problem you can also approximate your equation to look like 0.00675=x^2/0.177 because the Ka is small. This should make solving easier, good luck!

[Tony Chen 1F]

I don't think you can approximate x in this case because Ka is not less than 10^-3

[Madelyn_Rios_2c]

In this case, we approximate Ka. So you should use 0.00675= x^2 / 0.177 which ends up being x= 0.03456

[Jacob Bright 1C]

If you solved the problem by assuming that x was small enough to set (0.177 - x) to just 0.177, then I believe you will get an incorrect answer as the Ka value is not small enough to allow for such an approximation. In such scenarios, the value of Ka should usually be smaller than 1.0 x 10^-3 in order to be small enough to approximate. In this problem, you should instead use the quadratic equation to solve for the value of x.

Hope this helps!

[Saebean Yi 3E]

My problem had slightly different values, but your steps look correct to me. The first thing that should strike you with your answer is that an ionization percentage of 69.77, is unlikely as we are dealing with a weak acid.

Looks like you might have messed up your algebra/quadratic formula, because I am getting a different x value. Remember, you can't think of the x value as negligible if x is less than 5% of the concentration.

[Aparna Pillai 1E]

Your method and steps appear to be correct! I would double check your formula for the quadratic equation and how you entered your numbers in your calculator, since one or more of your calculations seem to be off.

[Shannon Clark 1F]

Hi Abby!
The steps and their order are all done correctly, the part where you made a mistake is with finding the value of x. For this problem, you x value should be x=0.031358. You should get the right answer if you use this x value instead for the rest of the calculations! :)

[Caroline 2A]

Hi!

First, start by assembling an ICE table. Because the value of M is more than 1000 times greater than the Ka, we are able to assume that the concentration change x is sufficiently small compared to the initial concentration. We can check this by dividing the value of X by the initial concentration and multiplying that value by 100%. If x is less than 5% of the concentration, then our assumption is valid (and it is valid in this case).

From there, we can find the pH by using the equation pH= -log[H+] where x is equal to H+. This should give you your final answer.

Hope this helps :)