The value of Kw for water at body temperature (37°C) is 2.1 X 10^-14. (a)What is the molarity of H3O+ ions and the pH of neutral water at 37°C? (b) What is the molarity of OH- in neutral water at 37"C?
For part a, I understand that Kw=[H3O] [OH-]
and were given Kw so 2.1 x 10^-14=[H30] [OH-]. I don't know where to go from here with these two pieces of information.
For part b, I understand if I get [H3O] solving for [OH] would entail dividing Kw/[H3O]
Value of Kw question
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Re: Value of Kw question
In water we assume that the concentration of [H3O+] = [OH-]
(such as when Kw = 1.0 x 10^-14, [H3O+] = 1.0 x 10^-7 and [OH-] = 1.0 x 10^-7)
so Kw = 2.1 x 10^-14 = [H3O+][OH-]
but since [H3O+] = [OH-]
Kw = 2.1 x 10^-14 = x^2
so x = square root 2.1 x 10^-14
Then, you get the pH by taking the -log of x.
Part b is simple since you already assumed [H3O+] = [OH-].
[OH-] = x
(such as when Kw = 1.0 x 10^-14, [H3O+] = 1.0 x 10^-7 and [OH-] = 1.0 x 10^-7)
so Kw = 2.1 x 10^-14 = [H3O+][OH-]
but since [H3O+] = [OH-]
Kw = 2.1 x 10^-14 = x^2
so x = square root 2.1 x 10^-14
Then, you get the pH by taking the -log of x.
Part b is simple since you already assumed [H3O+] = [OH-].
[OH-] = x
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Re: Value of Kw question
Franca Park 3J wrote:In water we assume that the concentration of [H3O+] = [OH-]
(such as when Kw = 1.0 x 10^-14, [H3O+] = 1.0 x 10^-7 and [OH-] = 1.0 x 10^-7)
so Kw = 2.1 x 10^-14 = [H3O+][OH-]
but since [H3O+] = [OH-]
Kw = 2.1 x 10^-14 = x^2
so x = square root 2.1 x 10^-14
Then, you get the pH by taking the -log of x.
Part b is simple since you already assumed [H3O+] = [OH-].
[OH-] = x
[H3O+] = [OH-] is simply because it is water? Or is it because the question stated neutral water?
Re: Value of Kw question
This is an old thread but I'm wondering the same thing– how do we know that H30=OH?
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Re: Value of Kw question
I'm also wondering why they would be equal...any help would be appreciated.
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Re: Value of Kw question
They would be equal because the problem claims the water is neutral. When water is neutral [H30+]=[OH-].
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Re: Value of Kw question
Kai_Chiu 1F wrote:Franca Park 3J wrote:In water we assume that the concentration of [H3O+] = [OH-]
(such as when Kw = 1.0 x 10^-14, [H3O+] = 1.0 x 10^-7 and [OH-] = 1.0 x 10^-7)
so Kw = 2.1 x 10^-14 = [H3O+][OH-]
but since [H3O+] = [OH-]
Kw = 2.1 x 10^-14 = x^2
so x = square root 2.1 x 10^-14
Then, you get the pH by taking the -log of x.
Part b is simple since you already assumed [H3O+] = [OH-].
[OH-] = x
[H3O+] = [OH-] is simply because it is water? Or is it because the question stated neutral water?
Water is always neutral unless it says otherwise so those would be the same!
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Re: Value of Kw question
Kw is the ionization constant of water. Ionization means the gain or loss of an electron from a molecule. In the case of water, the molecules that are losing electrons are H3O+ and OH-, and they are losing/gaining an electron in the form of a hydrogen ion. Water is a neutral substance. On the PH scale from 0 which is the most acidic, to 14, which is alkaline or basic, it measures at a 7 and thus is neutral. What does this mean? This means that the hydroxide and hydronium ions are essentially equal in concentration in water. So, if we are given the ionization constant, all we must do is take the square root of the value for the constant, which will tell us the concentration of each molecule! I hope this helps!
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