Please Help! Precipitation and solubility.

[JohnDoeDDS]

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Hello everyone, I need your help badly. Have an exam in Chem II coming up and don't understand how to do this problem at all. It's not in the text so I cant figure out how to do it. I would really appreciate it if you could help me. Okay here is the question:

If 1.00mL of AgNO3 and 1.00 mL Na2CO3 are mixed and [H+]=1.00M, what is the [CO3 2-] in solution? Will a precipitate form?

Ksp(Ag2CO3)=8.1X10^-12
Ka1(H2CO3)=4.2X10^-7
Ka2(H2CO3)=4.8X10^-11

Thank you so much for your help. Please also explain how you got to your answer. Thanks again everyone for your time.

 

[dat_student]

Hello everyone, I need your help badly. Have an exam in Chem II coming up and don't understand how to do this problem at all. It's not in the text so I cant figure out how to do it. I would really appreciate it if you could help me. Okay here is the question:

If 1.00mL of AgNO3 and 1.00 mL Na2CO3 are mixed and [H+]=1.00M, what is the [CO3 2-] in solution? Will a precipitate form?

Ksp(Ag2CO3)=8.1X10^-12
Ka1(H2CO3)=4.2X10^-7
Ka2(H2CO3)=4.8X10^-11

Thank you so much for your help. Please also explain how you got to your answer. Thanks again everyone for your time.

seems a bit strange. You need molarity, number of moles or mass of AgNO3 and Na2CO3.

 

[JohnDoeDDS]

oops sorry. This was part B of a question. Part A says

Molarity of AgNO3 =.2
Molarity of NA2CO3 = .2

 

[dat_student]

oops sorry. This was part B of a question. Part A says

Molarity of AgNO3 =.2
Molarity of NA2CO3 = .2

OK, you need to set up the equation for Ag2CO3 because this is the only thing that can precipitate out. If the product of your solubilities (molarities) is greater than the solubility product it precipitates out. I'll try to send you the solution. Right now, I need to get in touch with my proferssors and finish the rest of my application. It may take me a while to get back to you.

 

[dat_student]

Almost done with my application...Took forever to enter 80 courses

I am not so good at explaining but here is my solution:

If you set up the equation for Ag2CO3 you'll see that

(0.1 M)^2(0.1 M) > 8.1X10^-12 so a precipitate will form

again use the same rxn to find [CO3] in solution:

x = [CO3]
x^3 = (8.1 x 10^-12) / 4
x = [CO3] = 1.28 x 10^-4

 

[JohnDoeDDS]

grrr damnit I still dont gte it. Thanks for your help in taking the time but you think you can write the equation out? Where did you gte .1 for the molarity? also why did you need the .2 moles of AgNO3 and Na2CO3, and how did you use the .2. I dont see it anywhere. Thank you soo much. My exam is wednesday and I am freaking out because I don't know how to solve this!

*EDIT* Also I thought this was a problem where you combine the 3 equations like (Ka1*Ka2)/Ksp or something like that? Ughhh I am so confused. Welp!

 

[dat_student]

*EDIT* Also I thought this was a problem where you combine the 3 equations like (Ka1*Ka2)/Ksp or something like that? Ughhh I am so confused. Welp!

In this case, AgNO3 and Na2CO3 look very soluble & seem to dissociate completely in the final solution. In other cases, you may want to use some other Ks & combine equations. Had we had partial dissociation it would have been a different story. I don't see why in this case you need to do that.

 

[JohnDoeDDS]

okay thanks. But how do you know that it completly dissociates? Also could you please answer my question in the previous post about writing the equation out? Basically you are saying that I only need the Ksp, and that Ka1 and Ka2 are irrelevant in this case?

 

[dat_student]

okay thanks. But how do you know that it completly dissociates? Also could you please answer my question in the previous post about writing the equation out? Basically you are saying that I only need the Ksp, and that Ka1 and Ka2 are irrelevant in this case?

I think I made a mistake. I didn't notice [H+] is much bigger than [CO3] (1 M > 0.1 M). Now, it looks like no precipitate will form and you'll need to use your Ka's. Bicarbonates and carbonic acids don't dissociate a lot to flip the balance towards Ag2CO3 formation.

(1) Ag2CO3 <-> 2 Ag+ + CO32-
(2) H2CO3 <-> HCO3- + H+
(3) HCO3 <-> CO3 + H+

Now, the question looks more complicated than I originally thought.
Now, I don't think rxn 1 has much effect because [H+] is much bigger than [CO3]. To guesstimate [CO3] I'd use rxn 2 & rxn 3. Interesting question indeed ! 😉

 

[JohnDoeDDS]

damn it. Cmon guys. My exam is tomorrow lol. No1 knows how to do this? Aghhh.