## Kw equation

jordanginyard_
Posts: 81
Joined: Wed Sep 30, 2020 9:43 pm

### Kw equation

Is there a simpler way to explain Kw and how the 10^14 comes into play?

Aishwarya Kosgi 1F
Posts: 92
Joined: Wed Sep 30, 2020 10:00 pm

### Re: Kw equation

Hi! Kw is basically the Kc value for water. 2 H20 dissociates into [H30+] and [OH-] and the equilibrium constant for this reaction is called Kw, and we know the value of this is always 1x10^-14. This value is important because when you use pKw (taking the -log of Kw), you get 14, which is the sum of pH and pOH. If you have either the pH or pOH, you can calculate the other by using the equation 14=pH+pOH. Hope this helps!

Vince Li 2A
Posts: 95
Joined: Wed Sep 30, 2020 9:37 pm

### Re: Kw equation

Kw is the equilibrium constant for water, which is given to be 1x10^-14. I also just want to add that the reaction of autoprotolysis generates equal concentrations of H3O+ and OH-. Together, the two add to 14, which makes sense given the equilibrium constant for water. The main issue that comes with acids and bases are that there are other reactions that change the hydronium concentrations and hydroxide concentrations, which is where you have to do further calculations.

Sharon Kim 2A
Posts: 93
Joined: Wed Sep 30, 2020 9:42 pm

### Re: Kw equation

I like to think of it by referring back to the pH scale which was out of 14. Since half the pH scale is either acid or base, [H30+] and [OH-] would be 1x10^-7. Multiplying both the K values gives 1x10^-14. Kw comes into play especially in problems where it gives you the Kb value and need to convert to Ka since the problem usually asks for the pH. Kb will gives you the [OH-] so Kw will be needed to convert to Ka.

Edison Tham 3D
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### Re: Kw equation

I agree with the replies above; one thing to note is that Kw is only 10^-14 when the temperature is at 25 degrees Celsius.

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Joined: Thu Dec 17, 2020 12:19 am

### Re: Kw equation

It is important to note that recognizing what Kw is allows us to understand the relationship between acids and bases as conjugates! A solution with a higher Ka must have a lower Kb since the product of Ka and Kb must equal Kw.

Kandyce Lance 3E
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Joined: Wed Sep 30, 2020 9:46 pm

### Re: Kw equation

Edison Tham 3D wrote:I agree with the replies above; one thing to note is that Kw is only 10^-14 when the temperature is at 25 degrees Celsius.

Is there a constant value for Kw at other temperatures that we will be using in chem? Or is Kw at 25 degrees Celsius the only constant we know so if the example given isn't at 25 degrees celsius we'll have to adjust the way we approach the problem?

Chinyere Okeke 2J
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### Re: Kw equation

Kandyce Lance 3E wrote:
Edison Tham 3D wrote:I agree with the replies above; one thing to note is that Kw is only 10^-14 when the temperature is at 25 degrees Celsius.

Is there a constant value for Kw at other temperatures that we will be using in chem? Or is Kw at 25 degrees Celsius the only constant we know so if the example given isn't at 25 degrees celsius we'll have to adjust the way we approach the problem?

In my discussion, the TA mentioned that Kw does change as temperature changes. This is true as when temperature increases, the pH of neutral water decreases. And as temperature decreases the pH of neutral water increases.

For example, the Kw at 37-degrees C is 2.1x10^-14.

JoshMoore2B
Posts: 90
Joined: Wed Sep 30, 2020 9:51 pm

### Re: Kw equation

jordanginyard_ wrote:Is there a simpler way to explain Kw and how the 10^14 comes into play?

I haven't seen this mentioned yet completely, but Kw is the product of Ka and Kb in water. Ka = 1.0 x 10^-7 in water, and Kb = 1.0 x 10^-7 in water. This is why water is considered neutral, because Ka = Kb. Multiplying both (Ka x Kb) gives 1.0 x 10^-14, which is Kw. You can always find Ka or Kb from the other because Ka x Kb = Kw = 1.0 x 10^-14 is always true at 25 degrees Celsius, which is a very handy equation.

It is, in many senses, a very abstract concept. At its core, Kw measures the acidity and basicity of water, and explains why it is neutral.

rhettfarmer-3H
Posts: 101
Joined: Wed Sep 30, 2020 9:59 pm

### Re: Kw equation

in a short way, Kw is the Kc when water breaks down into H30 and OH. Therefore, this is extremely important. It creates the PH and POH scale it also allows us to convert from KA to KB and visa versa when we need it. SO therefore it is very important and it defines the whole base and acids concepts. 10^-14 is important for ph scale that goes from 1-14 and it also makes the equation KA*KB=Kw.