Approximation

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Cienna Henry 1J
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Approximation

Postby Cienna Henry 1J » Sun Mar 10, 2019 5:24 pm

When can we use the approximation that allows us to bypass calculating a quadratic?

Jenna Salas 2H
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Joined: Fri Sep 28, 2018 12:22 am

Re: Approximation

Postby Jenna Salas 2H » Sun Mar 10, 2019 5:31 pm

You can use approximation when the concentration you calculate is less five percent of the initial concentration.

David S
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Re: Approximation

Postby David S » Sun Mar 10, 2019 5:52 pm

You can only use the approximation when equilibrium constant K is notably small. In class, lavelle told us that in general, when K < 10^-3, we can assume the amount of reactant lost to form product (x) is small enough to ignore.

After using this approximation to solve for x, you have to check it using the 5% rule, which is


Example:

For the reaction with K = 1.64x10^-4 @Temp = 400K
lets pretend initial concentration of N2= 0.1, H2= 0.2, and NH3 = 0.

We know that

When we set up our Equilibrium equation and ice table, we get
That equation looks nasty, but since K is < 10^-3, we can simplify it to

when you solve for x, you want to check that your approximations were valid, so you do:
and
As long as those inequalities are true, you are good to go.

Mark 1D
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Joined: Fri Sep 28, 2018 12:18 am

Re: Approximation

Postby Mark 1D » Sun Mar 10, 2019 7:37 pm

Approximate if it follows 5% rule or equilibrium constant is below x10^-3. But I sometimes use quadratic anyway to double check. :)

BenJohnson1H
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Joined: Fri Sep 28, 2018 12:17 am

Re: Approximation

Postby BenJohnson1H » Sun Mar 10, 2019 8:44 pm

Is it acceptable to not round, even when permitted to, on the test?

Hilda Sauceda 3C
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Joined: Fri Sep 28, 2018 12:24 am

Re: Approximation

Postby Hilda Sauceda 3C » Sun Mar 10, 2019 8:54 pm

yeah i have the same question, if we dont round will it be considered wrong?

Matthew Casillas 1C
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Joined: Fri Sep 28, 2018 12:21 am

Re: Approximation

Postby Matthew Casillas 1C » Sun Mar 10, 2019 9:10 pm

Are we allowed to approximate below 10^-4 or 10^-3?

Cienna Henry 1J
Posts: 61
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Re: Approximation

Postby Cienna Henry 1J » Sat Mar 16, 2019 7:36 pm

Matthew Casillas 1C wrote:Are we allowed to approximate below 10^-4 or 10^-3?

I think I heard 10^-3


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