Ok. First of all this is a homework problem so please provide your steps on how you solved this.

 

Calculate the [math]K_{a}[/math] for the following acids using the given information.

 

(a) 0.220 M solution of H3AsO4, pH = 1.50

 

 

 

so i know the concentration of H+ ions would be 10^-1.5 = 0.0316 M

 

I think the dilution of the H3AsO4 is:

H3AsO4 --> 3H + AsO4

 

now what?

You are going to be calculating K(a1), or the K of the first proton.

 

So your reaction is:

 

H3AsO4 -> H+ + H2AsO4-

 

Now if you have a reaction of:

 

A + B -> C + D

 

Then K = ([A]*)/([C]*[D])

 

Where [A] = concentration of A

From what I recall of chemistry, you can use the concentration of H+ you found and the equation you found ([ce]H3AsO4 -> 3H+ + AsO4-[/ce]) to determine the final concentration of [ce]H3AsO4[/ce] and [ce]AsO4-[/ce]. From those you should be able to find [math]K_a[/math].

 

Oh, yeah, and what big314mp said about the first proton... I had forgotten about that. Sorry.

 

big314mp: Shouldn't that be

 

[math]K_a = \frac{[A^a][b^b]}{[C^c][D^d]}[/math]

 

where a, b, c and d are the coefficients of A, B, C, and D, respectively?

woah woah woah!!

 

I though it was

 

[math]

K_a = \frac{[C^c][D^d]}{[A^a][b^b]}

[/math]

 

if so, then it should be

 

[math]

K_a = \frac{[H+]^2}{[H_3 AsO_4]}

[/math]

Yikes, you're right about that. However, are you sure it's [math][H+]^2[/math]? As big314mp points out, you only want the first proton.

I though it should be [H+]^2

because the concentration of the H+ and H3AsO4 should be equal in a weak acid. But even after I compute the answer, I keep getting it wrong.

because the concentration of the H+ and H3AsO4 should be equal in a weak acid.

 

Why do you say that? It depends on the [math]K_a[/math] of the acid.

Check you denominator. Are you doing:

 

[H3AsO4] - [H+]

 

Because, once the H3AsO4 deprotonates, it no longer counts as a reactant. So however much deprotonates needs to be subtracted from the initial concentration of H3AsO4.

i feel an ICE table coming on

Check you denominator. Are you doing:

 

[H3AsO4] - [H+]

 

Because, once the H3AsO4 deprotonates, it no longer counts as a reactant. So however much deprotonates needs to be subtracted from the initial concentration of H3AsO4.

 

YESS!!!!! I LOVE YOU MAN!!!!!!!!!!!!!!!!!!!!!! HEARTS ALL AROUND!!!!

 

 

thank you everyone!!!!

I'd strongly reccomend that this type of question be answered using an ICE table. It IS possible without but it's much clearer in a table format