## X was ignored

Andrea_Torres
Posts: 98
Joined: Sun Sep 15, 2019 12:15 am

### X was ignored

In my discussion, when solving for x after doing the ice table we got x^2/(.10-x). Then when we plugged in the Ka= 1.8 x 10^-5 so the equation was 1.80 x 10^-5 = x^2/.10. Why is it that the -x was ignored?

VPatankar_2L
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### Re: X was ignored

The -x was ignored because the value would have been so small that 0.1-x would be rounded to 0.1. If K is less than 10^-3, you generally ignore the -x and use the initial concentration without explicitly factoring in the change.

Callum Guo 1H
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### Re: X was ignored

The value of x changes 0.1 so little that it's negligible

kevinolvera1j
Posts: 103
Joined: Fri Aug 02, 2019 12:15 am

### Re: X was ignored

If you are unsure about the cutoff for ignoring X, you can generally ignore if K is less than 10^-3 but you can always ignore if less than 10^-5.

Viviana Velasquez
Posts: 97
Joined: Sat Jul 20, 2019 12:15 am

### Re: X was ignored

X was ignored since K was so small (less than 10^-3) and X would make such a small change to .10

kausalya_1k
Posts: 50
Joined: Wed Nov 14, 2018 12:23 am

### Re: X was ignored

x was ignored in this case because K is less than 10^-3 so you know that x is going to be too small for it to make a difference in that case. You can test to see if this approximation is accurate as long as your final answer is less than 5% of the initial

005324438
Posts: 51
Joined: Sat Aug 24, 2019 12:16 am

### Re: X was ignored

This is called the X is small approximation. Relative to the initial concentration, the change is so small, less than 10^-3 smaller, so we can treat it as the same number before and after without significant change. This makes our math much, much easier.

Charysa Santos 4G
Posts: 107
Joined: Wed Sep 18, 2019 12:21 am

### Re: X was ignored

If the given K is less than 10^-3, then you can use approximation, meaning that you can disregard the X because it is small enough that it would not profoundly impact your equilibrium expression.

SarahCoufal_1k
Posts: 102
Joined: Thu Jul 25, 2019 12:17 am

### Re: X was ignored

when the K is less than 10^-3, the x subtracted. is going to make such a small difference it's acceptable and easier to just ignore it. Only ignore when it is addition or subtraction though, you should include it in multiplication or division I believe. If concentration. found is less than 5% of the initial concentration that also confirms it was ok to do the. approximation

Donavin Collins 1F
Posts: 66
Joined: Sat Sep 14, 2019 12:16 am

### Re: X was ignored

Since X had such a small value, removing it wouldn't change anything. The x wouldn't have had a big enough change on the 0.10.

Gabriel Ordonez 2K
Posts: 113
Joined: Sat Jul 20, 2019 12:15 am

### Re: X was ignored

The removal of the -x occurs because it is extremely small relative to the chemical equilibrium. We don’t say it is 0 because the numerator would be affected. We say it is small relative to the rest of the equilibrium allowing it to be ignored.

Noe BM 1J
Posts: 50
Joined: Sat Sep 07, 2019 12:17 am

### Re: X was ignored

The x value is so small that when you subtract it from .10 it doesn't make a big chante to it, so you can ignore it. When you ignore it, it becomes easier to solve for x because you would now have Ka = x^2/.10

Brian J Cheng 1I
Posts: 115
Joined: Thu Jul 11, 2019 12:15 am

### Re: X was ignored

X value was ignored because the K value was so small, meaning the amount of product formed compared to the reactants was very small. This means that the amount of products (x) can technically be ignored as a change factor which is why you can eliminate it.

Kevin Antony 2B
Posts: 99
Joined: Sat Sep 07, 2019 12:16 am

### Re: X was ignored

Because K is such a small value, < 10^-3, we can assume x will be so small that it is inconsequential. As a result, we ignore the -x in the denominator and solve as if it were not there.

Sydney Pell 2E
Posts: 100
Joined: Wed Sep 11, 2019 12:17 am

### Re: X was ignored

Why do we see if our approximation is correct by checking if the final answer is less than 5% of the initial? Is there a particular reason we use 5% as the cutoff?

Louise Lin 2B
Posts: 97
Joined: Sat Aug 17, 2019 12:15 am

### Re: X was ignored

the -x was ignored because the value is so small that it makes no difference to include it or not. This is called a small approximation.

IScarvie 1E
Posts: 66
Joined: Sat Aug 17, 2019 12:16 am

### Re: X was ignored

1.8x10^-5 expanded is the same thing as .000018. .1-.000018 = .09999 something, which is very very close to .1. So when x is very small, it will not affect the constant in any significant way

Nare Nazaryan 1F
Posts: 101
Joined: Fri Aug 09, 2019 12:17 am

### Re: X was ignored

X was ignored because 0.10 is less than 10^-3, which means it is so small that it would not make a difference to decrease by X; therefore, you only use the 0.10 (think of Dr.Lavelle's example of the million dollars minus \$10).

Mitchell Koss 4G
Posts: 128
Joined: Sat Jul 20, 2019 12:17 am

### Re: X was ignored

x is very small so it is basically zero for the denominator (0.0000000000000000001), but we leave x alone in the numerator cause having 0 up top would make the whole thing=0.

saigorijavolu2k
Posts: 108
Joined: Sat Sep 28, 2019 12:15 am

### Re: X was ignored

The x is ignored because it is such a small value, thus will not affect the answer extremely. When doing calculations with small x, make sure you do not just plug in 0, but rather emphasize that it is negligible not 0.

Posts: 45
Joined: Sat Aug 17, 2019 12:16 am

### Re: X was ignored

The -x can be ignored because it was small relative to the initial concentration.

stephaniekim2K
Posts: 79
Joined: Fri Aug 09, 2019 12:16 am

### Re: X was ignored

Since Ka is smaller than 103 you can ignore the x in the denominator. Since K is so small, x is negligible in this case and can be ignored for the sake of convenience of calculating.

Minh Ngo 4G
Posts: 137
Joined: Thu Jul 25, 2019 12:17 am

### Re: X was ignored

Because your Ka is less than 10^-3. Since Ka then is so small, you can ignore it for the denominator

nicole-2B
Posts: 103
Joined: Fri Aug 30, 2019 12:18 am

### Re: X was ignored

you usually ignore the -x when the Ka or Kb is less than 10^-3 because it has little effect to the concentrations

Joanne Lee 1J
Posts: 100
Joined: Thu Jul 25, 2019 12:15 am

### Re: X was ignored

If the Ka or Kb is less than 10^-3, then the x on the bottom can be ignored because the x value will be so small that it won't make a big difference to the original concentration.

Maya Beal Dis 1D
Posts: 100
Joined: Sat Aug 17, 2019 12:16 am
Been upvoted: 2 times

### Re: X was ignored

If you were unsure as to if x is a small enough value to ignore you could also just solve the problem as you usually would and get a correct answer it would just be more work.

JohannaPerezH2F
Posts: 94
Joined: Wed Sep 18, 2019 12:18 am

### Re: X was ignored

you can ignore x when making an approximation. this can be done when Kb or Ka is less than 10^-3

Celine 1F
Posts: 120
Joined: Fri Aug 02, 2019 12:16 am

### Re: X was ignored

The X can be ignored because it is so small and wouldn't cause a change in concentration