TBR Gen Chem Q: densities/mixtures/mole fractions

[capn jazz]

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From TBR:

Given two compounds, A and B.
B: higher molecular mass than A.
A: denser than B.
Which of the following mixtures has the greatest mole fraction of A?

A) 1.0 grams A with 1.0 grams B
B) 1.0 moles A with 1.0 moles B
C) 1.0 mL A with 1.0 mL B
D) 1.0 x 10^23 molecules A with 1.0 x 10^23 molecules B.

The answer is C. However, I don't see why C is a better answer than A.

A: If cmpd A has a lower MW than cmpd B, then a given mass of A and B will result in a higher # of moles for A than B, and thus a higher mole fraction.

C: If A has a higher density than B, then for a given volume of liquid, there will be more grams of A than B, and since A has a lower MW, those excess grams will result in a higher number of moles.

Actually, I think I just answered my question. For C, we're starting off with A already having a higher # of grams, which results in a higher # of moles. For A, they have the same #grams which still means A has a higher # of moles, but the difference between the #moles A and #moles B is smaller.

Is my reasoning correct?

 

[EkramVahsedi86]

From TBR:

Given two compounds, A and B.
B: higher molecular mass than A.
A: denser than B.
Which of the following mixtures has the greatest mole fraction of A?

A) 1.0 grams A with 1.0 grams B
B) 1.0 moles A with 1.0 moles B
C) 1.0 mL A with 1.0 mL B
D) 1.0 x 10^23 molecules A with 1.0 x 10^23 molecules B.

The answer is C. However, I don't see why C is a better answer than A.

A: If cmpd A has a lower MW than cmpd B, then a given mass of A and B will result in a higher # of moles for A than B, and thus a higher mole fraction.

C: If A has a higher density than B, then for a given volume of liquid, there will be more grams of A than B, and since A has a lower MW, those excess grams will result in a higher number of moles.

Actually, I think I just answered my question. For C, we're starting off with A already having a higher # of grams, which results in a higher # of moles. For A, they have the same #grams which still means A has a higher # of moles, but the difference between the #moles A and #moles B is smaller.

Is my reasoning correct?

Well, if you approach the problem analytically, you'll quickly realize that the mol fraction of answer choices B and D are simply 1:1

Since molecule A is denser than molecule B, then having "equal volumes" of each will favor a higher mass fraction of A.

Since A has lower molecular mass than B, then having equal masses of A and B will favor a higher mole fraction of A.

Answer choice C takes the most advantage of the two derived rules. Answer choice A only takes advantage of half of the newly derived rules.

If you put one gram of each substance into a testtube (as answer choice A might consider) then the more dense fluid will occupy less volume--so imagine you have two testtubes and the testtube of cmpd_B has slightly more fluid than cmpd_A. Now imagine if you can "top off" testtube A to make it the same volume. Obviously you added some more cmpd_A, and this topoff is the amount by which answer choice_C is better than answer choice_A.

For starters, call the compounds by better names: coke and pepsi. There is a lot of confusion due to recycling symbols.

From TBR:

Given two compounds, coke and pepsi.
pepsi has higher molecular mass than coke.
coke is denser than pepsi
Which of the following mixtures has the greatest mole fraction of coke?

A) 1.0 grams coke with 1.0 grams pepsi
B) 1.0 moles coke with 1.0 moles pepsi
C) 1.0 mL coke with 1.0 mL pepsi
D) 1.0 x 10^23 molecules coke with 1.0 x 10^23 molecules pepsi.

Now pretend you are a bartender and you just ran out of coke, and I ask for a coke and rum. Which beverage would I like best, and that would be answer choice C (because it has the highest proportion of coke molecules to pepsi molecules which will bind more frequently with my flavor receptors--I hate pepsi!)

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My signature = http://www.youtube.com/watch?v=CdD7l7htRac (check out the answer at 30 second mark = how many hours does the average person study for the mcat = 48)

 

[capn jazz]

Thanks a lot!