What Is the Initial Concentration of Na2CrO4 in a Mixed Ag2CrO4 Solution?

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Discussion Overview

The discussion centers around determining the initial concentration of Na2CrO4 in a mixed solution with Ag2CrO4, following the formation of a precipitate. It involves concepts from chemistry related to solubility product constants (Ksp), equilibrium concentrations, and dilution effects.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant presents a scenario involving the mixing of Ag2CrO4 and Na2CrO4 solutions and attempts to use an ICE table to find the equilibrium concentration of CrO4.
  • Another participant questions the final concentration of Ag+ after mixing, suggesting that Ksp should not be considered for this calculation.
  • A participant expresses confusion over the initial concentration of CrO4, leading to a negative value when calculated, indicating a potential issue with the problem setup.
  • Some participants discuss the implications of the negative concentration result, with one suggesting that the question may be flawed or misinterpreted.
  • There is a clarification that Ksp is not the same as solubility, which leads to further discussion about the definitions and implications of these terms.

Areas of Agreement / Disagreement

Participants express differing views on the validity of the problem statement and the calculations involved. Some agree that the negative concentration indicates a problem, while others suggest that the question might be acceptable under certain interpretations. No consensus is reached regarding the initial concentration of Na2CrO4.

Contextual Notes

Limitations include potential misunderstandings of solubility versus Ksp, and the assumptions made in the calculations that led to negative concentration values. The discussion reflects uncertainty regarding the problem's setup and the expected approach to solving it.

j3llzang
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Homework Statement


When 50.0 mL of a 2.0 × 10-4 M Ag2CrO4 solution was added to 50.0 mL of a Na2CrO4 solution a ppt formed (Ag2Cro4). What was the initial concentration of this Na2CrO4 solution?
Ksp Ag2CrO4 = 1.1 × 10-12

Homework Equations


Given A2 (aq) + B(aq) -> AB(s)
Ksp = [A]n

The Attempt at a Solution


I tried using the "ICE" Table, but it didn't help:
Ag2CrO -> 2Ag + CrO4
I 2.0x10-4 2.0x10-4 ?
C +x(?) -2x(?) -x(?)
E 2.0x10-4(unchanged) 2.75x10-5

I found [CrO4] at eqb using Ksp equation. (2.75x10-5M)
The initial [CrO4] was the [CrO4] from Ag2CrO4 plus Na2CrO4, right?
but why is it that [CrO4] at equilibrium is smaller than that of the initial [CrO4] in the initial Ag2CrO4 concentration?
Can anyone solve this problem?
Explanation would be nice :)

Thanks!~
 
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What was the final concentration of Ag+ after solutions has been mixed? (Don't think about Ksp, it was just diluted).

Can you calculate concentration of chromate now?
 
Borek said:
What was the final concentration of Ag+ after solutions has been mixed? (Don't think about Ksp, it was just diluted).

Can you calculate concentration of chromate now?

I like how everyone answers the questions with another question. lol

As I stated above, I did find the final concentration of Ag+, which came out to be 2.0x10-4M
 
I like how everyone concentrates on the first part of the answer but ignores the other part, one that leads to the solution.

So you have a 2.0x10-4M solution of Ag[sup+[/sup] and you know concentration of CrO42- is high enough to start precipitation. What is concentration of CrO42-? Where did it came from?
 
Hmm.. concentration of CrO4 comes from Na2CrO4...
Well, I used algebra to find the initial concentration of CrO4, and ended up with a negative value.
Here's what I did:
I declared x as [CrO4]init,
so [CrO4] @ eqb = 1.0x10-4 + x

Since Ksp = [Ag]^2[CrO4],
[CrO4] (at eqb?) = 1.1x10-12 / (2.0x10-4)^2
x = -7.25x10^5 M = [CrO4] initial.

I have no clue as to what this value actually means...
Any idea?
Thanks for your help :D
 
LOL, unless I am missing something that's not your fault. Question is wrong. There is no such thing as 2x10-4M solution of Ag2CrO4, no wonder you are getting negative concentrations.

Check what is silver chromate solubility.

On the second thought, there is a slight chance that question is OK and you were expected to take into account chromate protonation. But I strongly doubt it.
 
oh my gosh, that's so funny! I thought my chemistry teacher was evil enough to give us a question that had a negative answer :)
Well, solubility of Silver Chromate is 1.1x10-12, so I guess the question had it right.
But how do you know that 2.0x10-4 M solution doesn't exist?
Thanks!
(ps: I don't think we know what protonation is yet...)
 
j3llzang said:
Well, solubility of Silver Chromate is 1.1x10-12, so I guess the question had it right.

This is not solubility, this is Ksp. Solubility is a concentration of a saturated solution.
 

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