Question

A solution of 0.18mol of the chlorine salt of protinated atropine (AH+), a weak organic base, in 1.0L of solution has pH = 5.20.

Find the Kb of atropine (A).
I would like a hint.

Answer 1

are you familiar with a RICE box?

Answer 2

I am familiar with the ICE table.

Answer 3

A + H2O <=> AH+ + OH-
AH+ + H2O <=> AH2+ + OH-?

Answer 4

So I have to work backwards here?

Answer 5

use the protonated form and notice that the pH is given?

Answer 6

I know the pOH is going to be 8.8

Answer 7

AH+(aq) <--> A(aq) + H+(aq)

Answer 8

So using that, I can retrieve the value for [OH-]

Answer 9

Really?

Answer 10

pOH isn't important yet

Answer 11

Ok then the equation is really:
AH+ + H2O <=> A + H3O+

Answer 12

So then I can use pH to find [H3O+]?

Answer 13

You know the pH, so you can find the [H+]. the concentration of [A] must be equal to the [H+], since the ratio is 1:1. This gets you the KA of the protonated acid.

Do you know the relation between KA and KB?

Answer 14

Yes.

Answer 15

if you know the KA of a weak acid, you can find the KB of its conjugate weak base.

Answer 16

What is meant by protinated? How come you didn't decide to use the + H2O equation?

Answer 17

So I meant really, what does it mean in this context?

Answer 18

AH+ is the "protonated" base, making it act like an acid. You don't need to include the water, since the whole thing is surrounded by water anyway

Answer 19

Oh so it will act like an acid because if the equilibrium goes reverse, it will give up its H+.

Answer 20

essentially, yes

Answer 21

So, when they say "protinated", they really meant hat the organic base is just ionizing within the water, and so its ions will be on the RHS of the equation?

Answer 22

not necessarily, it just means that the base has accepted a proton from somewhere (usually water), and has the general formula AH+. Whether it's on the RHS or LHS doesn't matter. What DOES matter, is that the AH+ will act just like any other acid and partially dissociate forming H+ ions.

Answer 23

OHH. So that is why they say the base (A) has been protinated. Also, The Ka would just been retrieved from [H+][A]/[AH+], where [AH] = 0.18M?

Answer 24

correct

Answer 25

No, but would not the [] provided by subtracted from the concentration of H+?

Answer 26

be*

Answer 27

since the acid is weak, the change in concentration will be really small anyway, and we usually ignore it. Consider \[0.18M - 0.000001M \approx 0.18M \] it's like taking a teaspoon out of a dump truck full of dirt and asking if the weight has changed.