when is x negligible

Acidity
Basicity
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005199302
Posts: 108
Joined: Fri Sep 28, 2018 12:15 am

when is x negligible

Postby 005199302 » Thu Jan 17, 2019 9:46 pm

When can we assume that [OH-]=[H30+]? Is it when Kw is less than 10^3?

Parth Mungra
Posts: 72
Joined: Fri Sep 28, 2018 12:28 am

Re: when is x negligible

Postby Parth Mungra » Thu Jan 17, 2019 9:49 pm

x is negligible when the acid/base reacting with water is weak, meaning very little will dissociate into ions. Therefore, we consider this change in concentration (x) from initial to final to be so small that it can be negated.

anthony_trieu2L
Posts: 60
Joined: Fri Sep 28, 2018 12:29 am

Re: when is x negligible

Postby anthony_trieu2L » Thu Jan 17, 2019 9:54 pm

[OH-] is equal to [H30+] when the pH of the solution is neutral. Kw is always 1 x 10^-14 at 25 C.

Brandon Mo 4K
Posts: 70
Joined: Fri Sep 28, 2018 12:15 am

Re: when is x negligible

Postby Brandon Mo 4K » Fri Jan 18, 2019 3:05 am

You can also tell if x is negligible by seeing if the value of x is less than 5% of the initial molarity of the reactant after using the approximation.

Aria Soeprono 2F
Posts: 64
Joined: Fri Sep 28, 2018 12:27 am

Re: when is x negligible

Postby Aria Soeprono 2F » Fri Jan 18, 2019 8:49 am

Also, just something to keep in mind, is that if you have already solved the problem using the shortcut (when you assume x to negligible), Lavelle said that the general rule is anytime x is less than 5% of the initial value, then it is safe to assume an approximation to 0.

Marina Gollas 1A
Posts: 36
Joined: Fri Sep 28, 2018 12:20 am

Re: when is x negligible

Postby Marina Gollas 1A » Fri Jan 18, 2019 8:51 am

Here something that another student said in this forum that helped me understand. :)
"The reason that you can do an approximation where you "ignore" the x in the denominator [of the equation to find x when you complete the Ice tables], is because if the K value is smaller than 10^-3, then the x when multiplied out by this k value will be very negligible [since it's like 3-0.00000999, in which case the result is still quite close to 3, so there was barely any significance with this minimal change in the value]. So, instead of going through the whole process and doing the quadratic formula in this specific case, it is much easier and still pretty accurate to approximate the value of x in the denominator to be equal to zero."

LedaKnowles2E
Posts: 62
Joined: Fri Sep 28, 2018 12:27 am

Re: when is x negligible

Postby LedaKnowles2E » Fri Jan 18, 2019 9:40 am

X is negligible when x is less than 5% of the initial concentration, or when K is less than 10^-3.

Nathan Mariano 2G
Posts: 68
Joined: Fri Sep 28, 2018 12:29 am

Re: when is x negligible

Postby Nathan Mariano 2G » Sat Jan 19, 2019 6:39 pm

X is negligible when the equilibrium constant/acid dissociation constant (Ka)/base dissociation (Kb) constant is smaller than 10^-3. Also, as repeated above, X is negligible when it is less than 5% of the initial concentration.


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