## Sapling Week 2 #2

Acidity $K_{a}$
Basicity $K_{b}$
The Conjugate Seesaw $K_{a}\times K_{b}=K_{w}$

Racquel Fox 2I
Posts: 100
Joined: Wed Sep 30, 2020 10:11 pm

### Sapling Week 2 #2

At first when I did this problem, I made the assumption that you could change the 0.176-x in the denominator of the Ka equation to just 0.176 and got the answer wrong. When attempting it for the second time, I kept it as 0.176-x and got it right, but I'm a little confused on when you can assume that the -x isn't necessary.

Dylan_K_3B
Posts: 36
Joined: Wed Nov 18, 2020 12:24 am
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### Re: Sapling Week 2 #2

You can first try to make the assumption if your K value is less than 1 x 10^-4, and once you make that assumption and find x, you need to go back and make sure that x/initial concentration is less than 5%. However, if you do want to be extra safe, then not making the assumption will always get you the right answer.

Andreas Krumbein 1L
Posts: 69
Joined: Wed Sep 30, 2020 10:00 pm

### Re: Sapling Week 2 #2

The simplification of 0.176-x to 0.176 can only be made when the Ka or Kb is LESS than 10-3, and not less than or equal to, because even at 10-3, the simplification creates to much of an error between the real answer and the answer you would otherwise get.

Posts: 159
Joined: Wed Sep 30, 2020 9:32 pm
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### Re: Sapling Week 2 #2

A good rule to follow is when the K value is less than 10^-4. This means that there is a lot of the reactant present, so any small change will not make a significant impact on the concentration of the reactant. However, in this question the K value is much larger than that, so you cannot use the approximation.

Another way to check is to see if the x value (the concentration that you found) is truly negligible. To do this just divide the x value by the initial concentration that was used for the approximation (0.151 in this problem), and if the answer is less than 5%, then the approximation can be used. This fails for this problem so the approximation does not work.

I also just want to note that I believe the denominator for this question is 0.151, so that might have also played a role in the error.

Hope this helps!

Alejandro Gonzalez 2G
Posts: 77
Joined: Wed Sep 30, 2020 10:04 pm

### Re: Sapling Week 2 #2

I think the assumption can only be made when the K value is less than 10^-3, otherwise, you have to just use the normal method of finding x.