Kw constant

Moderators: Chem_Mod, Chem_Admin

Nane Onanyan 1G
Posts: 107
Joined: Wed Sep 30, 2020 9:43 pm
Been upvoted: 1 time

Kw constant

Postby Nane Onanyan 1G » Sat Jan 16, 2021 2:58 pm

When calculating Kw, why do we leave out [H2O]?

Kaitlyn Hernandez 3I
Posts: 135
Joined: Wed Sep 30, 2020 9:37 pm
Been upvoted: 2 times

Re: Kw constant

Postby Kaitlyn Hernandez 3I » Sat Jan 16, 2021 3:02 pm

I was curious about this question too. But I looked at my lecture notes, and since [H2O] is in large excess, we consider the concentration to be unchanged and therefore, we don't include it in the equation.

Nathan Lao 2I
Posts: 88
Joined: Wed Sep 30, 2020 9:40 pm
Been upvoted: 1 time

Re: Kw constant

Postby Nathan Lao 2I » Sat Jan 16, 2021 3:16 pm

I agree with the post above. The autoprotolysis of water is really small so it's almost negligible, and the H2O molecules remain in excess.

Lung Sheng Liang 3J
Posts: 90
Joined: Wed Sep 30, 2020 9:33 pm

Re: Kw constant

Postby Lung Sheng Liang 3J » Sat Jan 16, 2021 3:21 pm

We leave out H2O out of the calculations because the effect that it has is almost zero.

Eric Cruz 2G
Posts: 87
Joined: Wed Sep 30, 2020 9:45 pm
Been upvoted: 1 time

Re: Kw constant

Postby Eric Cruz 2G » Sat Jan 16, 2021 6:09 pm

we leave out h2o because there is an excess of it. It is both in the products and reactants so they cancel out.

manisha_joseph_1H
Posts: 92
Joined: Wed Sep 30, 2020 10:01 pm

Re: Kw constant

Postby manisha_joseph_1H » Sat Jan 16, 2021 6:25 pm

We leave out H2O because its concentration does not change significantly enough to where it needs to be considered in the calculation of the equilibrium constant.

Lucy Weaver 1K
Posts: 108
Joined: Wed Sep 30, 2020 9:56 pm

Re: Kw constant

Postby Lucy Weaver 1K » Sat Jan 16, 2021 6:32 pm

Since water is in large excess, we assume its concentration to be relatively unchanged. That is, we aren't able to detect the change in water since it's probably something in the decimal units so it is essentially undetectable.

DavidTabib 3H
Posts: 91
Joined: Wed Sep 30, 2020 10:06 pm

Re: Kw constant

Postby DavidTabib 3H » Sat Jan 16, 2021 6:36 pm

The H2O molecules are in excess which results in their effect being negligible.

Nayra Gharpetian 3F
Posts: 91
Joined: Wed Sep 30, 2020 9:40 pm
Been upvoted: 1 time

Re: Kw constant

Postby Nayra Gharpetian 3F » Sat Jan 16, 2021 6:45 pm

H2O is in excess so the change in concentration would be negligible. if we were to add it into Kw it would be in the numerator and denominator, and therefore would cancel out.

Neha Gupta 2A
Posts: 96
Joined: Wed Sep 30, 2020 9:52 pm

Re: Kw constant

Postby Neha Gupta 2A » Sat Jan 16, 2021 6:55 pm

You leave out H2O because it serves as a solvent. Because of this, the concentration is in excess and you don't need to include it in the equilibrium constant expression. The same thing would happen if you had any other substance as a solvent, such as ethanol in some reactions

Gerardo Ortega 2F
Posts: 101
Joined: Wed Sep 30, 2020 9:59 pm
Been upvoted: 1 time

Re: Kw constant

Postby Gerardo Ortega 2F » Sat Jan 16, 2021 7:00 pm

[H2O] is left out if water is the solvent. As Dr. Lavelle mentioned in yesterday's lecture, any other solvent such as ethanol would be left out of the equilibrium constant as well.

Kyle Dizon 3A
Posts: 96
Joined: Mon Oct 05, 2020 12:16 am

Re: Kw constant

Postby Kyle Dizon 3A » Sat Jan 16, 2021 8:21 pm

[H2O] will be left out due to the large excess which is why it can be disregarded due to its presence in both the reactant and products as stated in the lecture.

Annie Liang 3D
Posts: 40
Joined: Mon Jun 17, 2019 7:25 am

Re: Kw constant

Postby Annie Liang 3D » Sat Jan 16, 2021 8:38 pm

H2O is in large excess and is present in both the reactant and products, meaning there is a negligible change in concentration of water. As such, water is not included when calculating the equilibrium constant.

Ansh Patel 2I
Posts: 93
Joined: Wed Sep 30, 2020 9:42 pm

Re: Kw constant

Postby Ansh Patel 2I » Sat Jan 16, 2021 9:39 pm

Hi! H2O is left out of the calculations because it is present in excess and therefore the change in concentration is not significant.

Jason_John_2F
Posts: 91
Joined: Wed Sep 30, 2020 9:54 pm

Re: Kw constant

Postby Jason_John_2F » Sat Jan 16, 2021 10:04 pm

leave out the h20 because it will not have much of an effect on calculations

AustinMcBrideDis3L
Posts: 94
Joined: Wed Sep 30, 2020 9:39 pm

Re: Kw constant

Postby AustinMcBrideDis3L » Sat Jan 16, 2021 10:42 pm

If you don't want to understand why: just remember we always leave out solids and liquids, molecules with the (s) and (l) subscript. If you want to know why, its because the reaction is happening in a solution of water, that means there are TREMENDOUS amounts of water compared to the other molecules so if any water ends up being used up or created in a reaction it really will make no difference. It's like adding adding or removing a droplet of water in a pool.

Dominic Benna 2E
Posts: 93
Joined: Wed Sep 30, 2020 10:09 pm

Re: Kw constant

Postby Dominic Benna 2E » Sat Jan 16, 2021 10:58 pm

[H2O] is left out because it is found in large amounts on both the product and reactant side of the equation. This means, when calculating for Kw, the amounts of [H20] would just cancel each other out.

Gian Boco 2G
Posts: 86
Joined: Wed Sep 30, 2020 9:36 pm

Re: Kw constant

Postby Gian Boco 2G » Sun Jan 17, 2021 3:41 am

H2O is in excess amounts on both sides of the equation and can therefore be neglected as its minimal changes would not have any considerable effects

Will Skinner
Posts: 84
Joined: Wed Sep 30, 2020 9:28 pm

Re: Kw constant

Postby Will Skinner » Sun Jan 17, 2021 8:00 am

I think it is because H20 is present in excess and it’s change in concentration would be insignificant.

Carly_Lipschitz_3H
Posts: 99
Joined: Wed Sep 30, 2020 9:56 pm

Re: Kw constant

Postby Carly_Lipschitz_3H » Sun Jan 17, 2021 9:16 am

When calculating Kw, we leave out [H2O] because it is always present in excess in these reactions. As a result, any change would be negligible. Because of this, we leave it out when doing Kw.

OmarArafat_2K
Posts: 41
Joined: Thu Dec 17, 2020 12:17 am

Re: Kw constant

Postby OmarArafat_2K » Sun Jan 17, 2021 9:52 am

Yes we always leave out H2O from the calculations since it is in excess, so the change in concentration is not significant.

Krish_Ajmani_3J
Posts: 16
Joined: Wed Nov 20, 2019 12:20 am

Re: Kw constant

Postby Krish_Ajmani_3J » Sun Jan 17, 2021 9:55 am

The amount of H2O is high. As such, any change in concentration is negligible and does not need to be considered in calculations.

Vivian_Le_1L
Posts: 40
Joined: Thu Dec 17, 2020 12:19 am

Re: Kw constant

Postby Vivian_Le_1L » Sun Jan 17, 2021 7:30 pm

[H2O] is in excess. Because there is a lot of H2O a change in concentration would have minimal effect, and thus it is negligible.

905409193
Posts: 94
Joined: Wed Sep 30, 2020 10:06 pm

Re: Kw constant

Postby 905409193 » Sun Jan 17, 2021 7:35 pm

Since such a little amount of H2O is ionized, it's concentration will pretty much stay the same making it insignificant.

Griffin G
Posts: 89
Joined: Wed Sep 30, 2020 9:50 pm

Re: Kw constant

Postby Griffin G » Sun Jan 17, 2021 8:35 pm

A solvent like H2O is usually in such a large excess that it's affect on the system is so minimal it's a waste of time to include it.

Lizbeth Garcia 1F
Posts: 92
Joined: Wed Sep 30, 2020 9:39 pm

Re: Kw constant

Postby Lizbeth Garcia 1F » Sun Jan 17, 2021 8:43 pm

Like everyone has stated, it is because H20 is the solvent and thus in excess. This means that it will be on both sides of the reaction resulting in it canceling out during the calculations. However, water is not always left out only when it is the solvent.


Return to “Equilibrium Constants & Calculating Concentrations”

Who is online

Users browsing this forum: No registered users and 1 guest