X was ignored

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Andrea_Torres
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X was ignored

Postby Andrea_Torres » Sun Jan 19, 2020 6:00 pm

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

Postby VPatankar_2L » Sun Jan 19, 2020 6:22 pm

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

Postby Callum Guo 1H » Sun Jan 19, 2020 6:23 pm

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

kevinolvera1j
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Re: X was ignored

Postby kevinolvera1j » Sun Jan 19, 2020 6:26 pm

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
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Re: X was ignored

Postby Viviana Velasquez » Sun Jan 19, 2020 6:34 pm

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

kausalya_1k
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Re: X was ignored

Postby kausalya_1k » Sun Jan 19, 2020 6:37 pm

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
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Joined: Sat Aug 24, 2019 12:16 am

Re: X was ignored

Postby 005324438 » Sun Jan 19, 2020 7:15 pm

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
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Re: X was ignored

Postby Charysa Santos 4G » Sun Jan 19, 2020 7:20 pm

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
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Re: X was ignored

Postby SarahCoufal_1k » Sun Jan 19, 2020 7:24 pm

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
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Re: X was ignored

Postby Donavin Collins 1F » Sun Jan 19, 2020 7:26 pm

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
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Re: X was ignored

Postby Gabriel Ordonez 2K » Sun Jan 19, 2020 7:33 pm

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
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Joined: Sat Sep 07, 2019 12:17 am

Re: X was ignored

Postby Noe BM 1J » Sun Jan 19, 2020 8:58 pm

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
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Re: X was ignored

Postby Brian J Cheng 1I » Sun Jan 19, 2020 9:02 pm

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
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Joined: Sat Sep 07, 2019 12:16 am

Re: X was ignored

Postby Kevin Antony 2B » Sun Jan 19, 2020 9:02 pm

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
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Re: X was ignored

Postby Sydney Pell 2E » Sun Jan 19, 2020 9:11 pm

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
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Re: X was ignored

Postby Louise Lin 2B » Sun Jan 19, 2020 9:50 pm

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
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Re: X was ignored

Postby IScarvie 1E » Sun Jan 19, 2020 9:54 pm

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
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Re: X was ignored

Postby Nare Nazaryan 1F » Sun Jan 19, 2020 10:54 pm

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
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Re: X was ignored

Postby Mitchell Koss 4G » Mon Jan 20, 2020 8:04 am

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
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Re: X was ignored

Postby saigorijavolu2k » Mon Jan 20, 2020 9:53 am

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.

Ashley Alvarado 2C
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Re: X was ignored

Postby Ashley Alvarado 2C » Mon Jan 20, 2020 10:19 pm

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

stephaniekim2K
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Re: X was ignored

Postby stephaniekim2K » Tue Jan 21, 2020 1:22 am

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
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Re: X was ignored

Postby Minh Ngo 4G » Tue Jan 21, 2020 10:19 am

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

nicole-2B
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Re: X was ignored

Postby nicole-2B » Tue Jan 21, 2020 8:59 pm

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
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Re: X was ignored

Postby Joanne Lee 1J » Wed Jan 22, 2020 11:20 pm

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
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Re: X was ignored

Postby Maya Beal Dis 1D » Thu Jan 23, 2020 9:02 am

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
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Re: X was ignored

Postby JohannaPerezH2F » Thu Jan 23, 2020 9:19 am

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

Celine 1F
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Re: X was ignored

Postby Celine 1F » Thu Jan 23, 2020 10:18 am

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


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