What the... Berkley Review Stoichiometry question~~~

[johnwandering]

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There is this question that I am struggling with


"Given two compounds, Compounds A and compound B, and the fact that B has a higher molecular mass than A, but A is denser than B, which of the following mixtures would have the greatest mole fraction of A?"

A.) The mixture of 1.0 grams Compound A with 1.0 grams Compound B
C.) The mixture of 1.0 mL Compound A with 1.0 mL Compound B


I only put these two choices in because they are the important ones
But aren't both of them right??

 

[BerkReviewTeach]

There is this question that I am struggling with


"Given two compounds, Compounds A and compound B, and the fact that B has a higher molecular mass than A, but A is denser than B, which of the following mixtures would have the greatest mole fraction of A?"

A.) The mixture of 1.0 grams Compound A with 1.0 grams Compound B
C.) The mixture of 1.0 mL Compound A with 1.0 mL Compound B


I only put these two choices in because they are the important ones
But aren't both of them right??

The key fact is the relative densities of the two compounds.

A) equal masses of A and B
C) equal volumes of A and B which means greater mass of A than mass B

Choice C has a great percentage of A than choice A.

 

[johnwandering]

Oh, I get it

For some reason, I kept reading the question as "greater fraction of A" than greatest...

 

[BerkReviewTeach]

Oh, I get it

For some reason, I kept reading the question as "greater fraction of A" than greatest...

It's a damn tricky question that messes with most people.

The takehome message is that it really didn't matter what measurement they were using to measure the relative amounts of A and B, only the units of A and B in the mixture.

 

[Durbic]

It's a damn tricky question that messes with most people.

The takehome message is that it really didn't matter what measurement they were using to measure the relative amounts of A and B, only the units of A and B in the mixture.

Can you demonstrate the math that is involved with that question? I'm still perplexed by it. Much appreciated!

 

[chill3]

Can you demonstrate the math that is involved with that question? I'm still perplexed by it. Much appreciated!

Density = mass/volume

We know that:
Density(a) > Density(b)
MolarMass(a) < MolarMass(b) ..... g/mol(a) < g/mol(b) ...... mol(a)/g > mol(b)/g

1g A vs. 1g B:

mol(a)/g * 1g > mol(b)/g * 1g

Same mass, so A has more moles by the property of it having more moles per gram of sample.

1ml A vs. 1ml B

mass(a)/ml > mass(b)/ml
given equal volume, mass(a) > mass(b) AND mol(a)/g > mol(b)/g.

Essentially a double whammy. More of compound A and more moles per g, where as in the other choice you simple have the same mass with more moles per gram.

 

[osckey]

Can you demonstrate the math that is involved with that question? I'm still perplexed by it. Much appreciated!

"Given two compounds, Compounds A and compound B, and the fact that B has a higher molecular mass than A, but A is denser than B, which of the following mixtures would have the greatest mole fraction of A?"

A.) The mixture of 1.0 grams Compound A with 1.0 grams Compound B
C.) The mixture of 1.0 mL Compound A with 1.0 mL Compound B


So, this is a very tricky question. The questions asks for a scenario where the GREATEST mole fraction of A would exist. Although both of these scenarios present situations where mole fraction of A is larger than B, however one of them represents a scenario where mole fraction of A would be greatest.

Scenario A shows that each compound has equal number of grams and in this case, yes, mole fraction of A is greater than B since A has a lower molecular weight meaning that more moles can fit in a 1 gram sample than for B.

So, if we've established that at equal masses that A will have a higher mole fraction than B, wouldn't it stand to reason that if we had even greater mass for A that the we'd an even greater mole fraction? YES! I used the simple density equation: d=m/v. Given the same volumes, and knowing that A has a greater density than B, then we know that A's higher density has to come from a higher mass (given that volume is equal in both A and B in this scenario).

Therefore, we've established that A is present in greater mass than B in this scenario and we can say that this scenario is the one where the GREATEST mole fraction for A exists over B.

Tldr: If we know that in the same masses, A has a greater mole fraction than B, we know a scenario where A has more mass than B would would have a greater mole fraction of A.

 

[manohman]

There is this question that I am struggling with


"Given two compounds, Compounds A and compound B, and the fact that B has a higher molecular mass than A, but A is denser than B, which of the following mixtures would have the greatest mole fraction of A?"

A.) The mixture of 1.0 grams Compound A with 1.0 grams Compound B
C.) The mixture of 1.0 mL Compound A with 1.0 mL Compound B


I only put these two choices in because they are the important ones
But aren't both of them right??

Bumping this because it tripped me up as well.

Why doesnt the whole "1 mole of gas = 22.4L" rule apply here? Because then the number of moles of A equal the number of moles of B.

How do you know when to take 22.4 L per mole of gas into account?

 

[Quinoline]

Bumping this because it tripped me up as well.

Why doesnt the whole "1 mole of gas = 22.4L" rule apply here? Because then the number of moles of A equal the number of moles of B.

How do you know when to take 22.4 L per mole of gas into account?

I believe the reason why you can't use 1 mol = 22.4 L is because this is only valid for ideal gasses at standard temperature and pressure. The question states "compound A, compound B," not "ideal gas A, ideal gas B at STP."