G-chem questions.. help!

[impeterle]

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1. 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 one is this
EXCEPTION?
A. Precipitation
B. Neutralization
C. Evaporation
D. Decomposition
E. Disproportionation

answer is A. Why? If a solute precipitates in the solvent, isn't the molar concentration of it decreasing?

If the mass percent of oxygen in a nitrogenoxygen
compound is known, given the mass
percent of oxygen, all of the following are
needed to determine the molecular formula of
a nitrogen-oxygen compound EXCEPT one.
Which one is this EXCEPTION?
A. Atomic mass of nitrogen
B. Atomic mass of oxygen
C. Avogadro's number
D. Empirical formula
E. Molar mass of the compound

 

[klutzy1987]

1) because how would precipitation make a higher concentration of a solvent. What happened here is that you started at 0 M of X an you are now at .4M x. You are going to end up at .6M meaning that at .4M it is not yet saturated. So your concentration went up 0-->.4 and its not saturated so it is impossible for the reason for this to be percipitation.

 

[klutzy1987]

2) Where should Avogadros number come in in this question. You need the atomic masses so that you can calculate the moles. Once you have the moles you are going to figure out the empirical formula. Once you have the empirical formula you would use the molar mass to divide and solve for the molecular formula.