Gen Chem question !!

[tokyoman]

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A 0.60 M solution is made by dissolving solid compound X in water. After ten seconds, the concentration of X is 0.40 M . All of the following could account for these results except one. Which is

A.Precipitation
B. Neutralization
C. Evaporation
D. Decomposition
E. Disproportionation

Ans : A

Can someone explain me ?
Should Precipitation increase the Conc of X because it forms back solid?

I understood neutralization and decomposition can lower the Conc of Solid X. But what about evaporation and disproportionation?

=( im so confused

by the way this is one of the OAT questions

 

[UCB05]

A 0.60 M solution is made by dissolving solid compound X in water. After ten seconds, the concentration of X is 0.40 M . All of the following could account for these results except one. Which is

A.Precipitation
B. Neutralization
C. Evaporation
D. Decomposition
E. Disproportionation

Ans : A

Can someone explain me ?
Should Precipitation increase the Conc of X because it forms back solid?

I understood neutralization and decomposition can lower the Conc of Solid X. But what about evaporation and disproportionation?

=( im so confused

by the way this is one of the OAT questions

I would have gone with C. Basically you have to identify what each of the processes can do to the concentration of X in solution and find the one that increases concentration of [X].

It seems unlikely to me that precipitation would be the correct answer since if precipitation does take place, it would reduce the amount of dissolved X and thus decrease [X]. One can make the stretch that since the solute was dissolved at a higher concentration, its saturation level is at least 0.6M, so precipitation would not take place, so it would not be the reason why concentration takes a nosedive. If precipitation does take place it does explain the drop in [X] and would be a wrong answer. If it doesn't take place, it would not explain the drop in [X] and would be the correct answer.

Neutralization and Decomp you understand already. Disproportionation, for the purpose of determining effect on concentration, would be similar to Decomposition. 2X -> X' + X''. Basically, [X] (the important one) decreases while [X'] and [X''] increase.

Like I said, I would have gone with C. Since the question states X is a solid, I would assume that evaporation would not be of solute X, but rather the liquid solvent. This would lead to an increase in [X] as the absolute amount of X remains the same but solvent decreases. It would therefore not account for the drop in [X].

C seems to be the most clear-cut answer. /shrug

 

[UndergradGuy7]

MY reasoning...
concentration is moles / liter

So
A.Precipitation - you lose some moles so the concentration can decrease...
B. Neutralization - X can turn into something else. Concentration can decrease.
C. Evaporation - The solution evaporates. So moles / L. Now you have less liters = higher concentration...

Didn't bother with the other 2... C seems reasonable.

 

[DATkiller]

I vote for C too. OP: where did you find this question?