Chemistry
posted by Xi .
Hmm... This does not make sense. I learned that anything over 5% error, we cannot accept so we have to use the quadratic formula.
Here is a question I got 5.831%. It didn't use the quadratic formula and it still said to set x=0.
Ka = 6.8x10^4
HF = 0.2x
H+ = x
F = x
x= 0.011662
I take 0.011662 and compare it to the original one which is 0.2.
0.011662/0.2 x100 = 5.83 %

Chemistry 
DrBob222
I don't understand your problem. It's over 5% so use the quadratic formula. I obtained 0.0113 which isn't that far from 0.01166 but 0.01133 satisfies the equation better than 0.01166. Technically, we're splitting hairs because you can use only 2 significant figures anyway since Ka isn't known any better than that. That being said, there isn't that much difference between 0.012 and 0.011.

Chemistry 
Xi
Are we allowed to set x = 0 if it is like 5.83%?

Chemistry 
DrBob222
Which x are you talking about?
The equation, I assume is
x^2/(0.2  x)] = 6.8 x 10^4.
You may NOT set the x of x^2 to zero else you can't solve the equation. Setting the x in the denominator to zero is allowed if the 5% rule (or whatever your prof sets), is not exceeded. In this case, it is exceeded so it is not allowed. [By the way, a better way of saying it is "assume 0.2x = 0.2" and that way it isn't confusing which x we are talking about.] And for whatever it's worth, I always told my students to use 10%; however, Ks can be determined with better accuracy now so the 5% rule is a little more strict. Technically, it doesn't matter much except where x is large compared to the molarity or where Ka is large such as 10^1 or 10^2. Another thing to look at is that often teachers want their students to get practice at (a) solving the quadratic and (b)determining when the quadratic is appropriate so setting a smaller percent gets those two purposes accomplished. You can see how much difference those two numbers make if you substitute 0.01133 for x (and use 0.2x) and see if the left side = 6.8 x 10^4, then use 0.01166 and do the same thing. When I did this I came up with about 7.22 x 10^4 when I used 0.01166 and it was 6.80 x 10^4 when I used 0.1133. That's a 5.8% error when using 0.01166 (but you already knew that). I hope this helps.
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