What is the [A2+] at Equilibrium When Mixed with B-?

Click For Summary

Discussion Overview

The discussion revolves around determining the equilibrium concentration of A2+ after mixing equal volumes of 3.60 M A2+ and 6.80 M B-. Participants analyze the problem using an ICE table and consider the implications of the reaction stoichiometry on the final concentrations.

Discussion Character

  • Homework-related, Debate/contested, Mathematical reasoning

Main Points Raised

  • One participant presents an ICE table approach to calculate the equilibrium concentration of A2+, arriving at a value of 1.47 M, which they believe is incorrect according to the answer key.
  • Another participant suggests an alternative reaction stoichiometry, proposing that the reaction should be A2+ + 2B- ⇌ AB2, which leads to a different calculation for equilibrium concentrations.
  • A third participant notes that without a specified reaction, the problem could have multiple interpretations, indicating uncertainty in the approach.
  • Further contributions clarify that the initial concentrations after mixing should be adjusted to 1.8 M for A2+ and 3.4 M for B-, emphasizing the importance of correct initial conditions.
  • Some participants express skepticism about the accuracy of the provided answer key, suggesting that it may contain errors.
  • One participant asserts that the specified reaction supports the answer given in the key, referencing another post for validation.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the correct stoichiometry of the reaction or the resulting equilibrium concentration of A2+. Multiple competing views remain regarding the interpretation of the problem and the calculations involved.

Contextual Notes

Participants note the lack of a specified reaction in the problem statement, which leads to ambiguity in determining the correct equilibrium concentrations. There are also discussions about the initial concentrations after mixing, which may affect the calculations.

brbrett
Messages
35
Reaction score
0

Homework Statement


Equal Volumes of 3.60 M A2+ and 6.80 M B- are mixed. After the reaction, equilibrium is established with [B-] = 0.40 M.
What is the [A2+] at equilibrium?

Homework Equations


Ice table, algebra

The Attempt at a Solution


This is probably a really easy problem, but I seem to be getting the wrong answer. Probably just a small lapse, but anyways, here's my work.

Equilibrium constant not given.

Ice Table:
A2+ (aq) + 3B- ==><== AB3-
I 3.60 6.80 0
C +x -3x +x
E 3.60 - x 0.40 +x

By subtracting 0.40 from 6.80, I determined 3x to be 6.4.
Using the ratios, I divided 3x by 3 to get 2.13(3 repeating).
Next I subtracted x from 3.60, and got my answer or 1.47.

The answer key says I am wrong, and that [A2+] at equilibrium is 0.80 M.

So am I going about this wrong, or is the book wrong (it has been in the past)?
Thanks!
 
Physics news on Phys.org
brbrett said:
A2+ (aq) + 3B- ==><== AB3-

Shouldn't it be $$A^{2+} + 2B^{-} \rightleftharpoons AB_2$$

$$\begin{array}[c!c!c!c!]
\text{ }& A^{2+} & 2B{-}&AB_2\\
\text{Initial concentration}& 3.60 & 6.80 & 0\\
\hline
\text{Change}& -x&-2x&x\\
\hline
\text{Equilibrium concentration }& 3.6 -x & 6.8 -2x=(0.40) & x\\
\hline
\end{array}$$Anyhow answer in books seems incorrect.
 
Last edited:
As long as the reaction is not given in the problem statement it can be everything.
 
It says "equal volumes of...are mixed". So immediately after mixing the initial concentrations are 1.8M and 3.4M. But as Borek says, if they do not specify the reaction there is no unique answer.
 
The reaction was specified in this case, and it is the one I posted above. This text has a lot of errors, so I'm not too surprised it's wrong this time. Good to double check though.
 
brbrett said:
The reaction was specified in this case, and it is the one I posted above.

Then the answer given is the correct one. See mjc123 post.
 

Similar threads

Replies
15
Views
3K
  • · Replies 3 ·
Replies
3
Views
3K
  • · Replies 4 ·
Replies
4
Views
2K
  • · Replies 1 ·
Replies
1
Views
2K
Replies
8
Views
2K
  • · Replies 2 ·
Replies
2
Views
2K
Replies
3
Views
2K
  • · Replies 4 ·
Replies
4
Views
5K
  • · Replies 13 ·
Replies
13
Views
7K
Replies
2
Views
4K