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[OATeater]

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Hey everyone,

I just practised myself on Chemistry from the opted site.. I got a 370 under "ideal conditions" (ie at home in my pyjamas haha).

There are a couple that are throwing me for a loop, I can't seem to figure out why I got them wrong:

1) Number 44: " IF the mass percent of oxygen in a nitrogen oxygen compund is known, given the mass percent of oxygen, all of the following are needed to determine the molcular 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) empirial formula
e) molar mass of the compound.'

and 2)

A 0.60 M solution is made by dissolvign solid compound X in water. After 10seconds, the concentraion of X is 0.40 M. All of the follwoing could account for tthese results EXCEPT ONE. Which on is this EXCEPTION?

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

The test says its

*Spoiler warning*














1) D - Empirical formula. I dont know how thats true..i certanly use Emp. formula do to my molecular formula calculation ... I would have said "C" avogadros's number.

2) A - Precipitation. I'm VERY confused at this one.

Any help will be rewarded with a cookie.

Cheers.

 

[gochi]

Hey everyone,

I just practised myself on Chemistry from the opted site.. I got a 370 under "ideal conditions" (ie at home in my pyjamas haha).

There are a couple that are throwing me for a loop, I can't seem to figure out why I got them wrong:

1) Number 44: " IF the mass percent of oxygen in a nitrogen oxygen compund is known, given the mass percent of oxygen, all of the following are needed to determine the molcular 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) empirial formula
e) molar mass of the compound.'

and 2)

A 0.60 M solution is made by dissolvign solid compound X in water. After 10seconds, the concentraion of X is 0.40 M. All of the follwoing could account for tthese results EXCEPT ONE. Which on is this EXCEPTION?

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

The test says its

*Spoiler warning*














1) D - Empirical formula. I dont know how thats true..i certanly use Emp. formula do to my molecular formula calculation ... I would have said "C" avogadros's number.

2) A - Precipitation. I'm VERY confused at this one.

Any help will be rewarded with a cookie.

Cheers.

Im just guessing.

1) I think the empirical formula tells you the multiples i,e CXYZ.. so if you have multiples that doesnt tell you or help you to determine the moelcular formula.

2) If the compound percipitated than it would be more concentrated leading to a higher molarity which the question does not state; its the reverse.

 

[OATeater]

1) Hmmm I dunno how you get molecular formulas but I take the Molar Mass of the Empirical formula, divide it by the grams of compound we'r talking about, and tha comes out as a whole number that you multiply the Emp. formula by:

1) Empirical Formula : HO = 17 g/mol
2) "We have 34 grams of this unknown substance..."
3) 34 / 17 = 2
4) 2HO = H2O2
5) We have peroxide.

Ain't that the case?

2) If something precipitates out, its no longer involved in the 'concentration.' Its like having a supersaturated solution of water and sugar. If you add more sugar to a saturated solution, the concentration doesn't go up. The sugar just sits at the bottom! (Temperature being constant)

 

[ToePUG]

I looked at it like this:

If you know the % mass of O in a oxygen-nitrogen compound, then you can figure out the % mass of N.

e.g., % mass O = 69.5%
% mass N = (100% - 69.5% = 30.5%)

If you then know the molar mass of the compound, you can use the % masses of O and N to determine the contribution of both elements.

e.g., molar mass = 46
O: .695 * 46 = 32
N: .305 * 46 = 14

Now to find the formula you need the atomic mass of O and N.

e.g., O: 32/16 = 2
N: 14/14 = 1

We now have the molecular formula (and empirical formula) and we didn't need the empirical formula. It's NO2.

But I didn't use Avogrado's number either... so I can't explain why that's not the answer. And besides which... I could be completely wrong...

 

[OATeater]

Hey I've been forgetting to say thanks. So thanks guys!


But yea, I understand what you did there -- but thats a special case where the molecular formula and the empirical formula are the same. The empirical formula being the smallest ratio of the different elements needed for the compound.

For example: CH20 is the empirical formula for sugar, but glucose is C6H12O6...


Any ideas for the second question btw?

Thanks!