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Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: Bigweight on December 03, 2019, 04:36:04 AM

Title: How do I calculate the concentration of a weak base from PH
Post by: Bigweight on December 03, 2019, 04:36:04 AM

Hello everyone, I have to calculate the concentration of Diethylammine (Kb 8.6 10^-4) only knowing the PH of the solution, which is 8.2

How do I do it? I tried to do

C= [OH]^2/Kb

But the result doesn't make much sense..

Title: Re: How do I calculate the concentration of a weak base from PH
Post by: mjc123 on December 03, 2019, 04:55:08 AM

That equation is an approximation that applies only when the degree of ionisation is small.
Suppose the degree of ionisation is x. Then the concentration of free base is C(1-x) and [BH⁺] = [OH^-] = Cx.
Can you work out what x and C are?

Title: Re: How do I calculate the concentration of a weak base from PH
Post by: Bigweight on December 03, 2019, 05:04:28 AM

Quote from: mjc123 on December 03, 2019, 04:55:08 AM

That equation is an approximation that applies only when the degree of ionisation is small.
Suppose the degree of ionisation is x. Then the concentration of free base is C(1-x) and [BH⁺] = [OH^-] = Cx.
Can you work out what x and C are?

I did the approximation for the reason that I can't work out X and C.

All I have is the PH of the solution and of course the Kb of Diethylammine

Title: Re: How do I calculate the concentration of a weak base from PH
Post by: AWK on December 03, 2019, 05:45:54 AM

Please, show your equation with numbers.

Title: Re: How do I calculate the concentration of a weak base from PH
Post by: Bigweight on December 03, 2019, 07:20:37 AM

Quote from: AWK on December 03, 2019, 05:45:54 AM

Please, show your equation with numbers.

Ok then:

PH starts at 8.2 meaning POH is 5.8

This means that [OH] is 10^-5.8

So the concentration of Diethylammine is

[D] = (10^-5.8 )^2/8.6 10^-4

Which is 2.9 10^-17 and it makes no sense

Title: Re: How do I calculate the concentration of a weak base from PH
Post by: mjc123 on December 03, 2019, 07:21:51 AM

Quote

I did the approximation for the reason that I can't work out X and C.

All I have is the PH of the solution and of course the Kb of Diethylammine

You have (modifying your equation above)
C(1-x) = [OH^-]²/K_b
and
Cx = [OH^-]
So you have two equations and two unknowns

Title: Re: How do I calculate the concentration of a weak base from PH
Post by: AWK on December 03, 2019, 07:25:15 AM

This result is wrong - calculation error.
Solve more exact equation K_b=[OH^-]²/(c-[OH^-])

Title: Re: How do I calculate the concentration of a weak base from PH
Post by: Borek on December 03, 2019, 08:38:53 AM

Quote from: Bigweight on December 03, 2019, 04:36:04 AM

C= [OH]^2/Kb

That's just an approximation. Each time you use an approximated equation you should check, if you are allowed to do so - that is, whether the approximation used when deriving holds.

Do you know how this equation is derived? What approximation is being used? Does it hold?