Berkley Review General Chem. Q 1.17

[mophead4]

If a molecule is composed of only two elements (X and Y), and if X and Y combine in equal mass quantities, and if Y is less than twice as heavy as X, which of th following molecular formulas is NOT possible?

A. XY

B. XY2

C. X3Y2

D. X3Y

So I was wondering if anyone could explain this question any differently than the solution in the book. I understand the concept of setting a hypothetical maximum, but I just don't get how A and B fit this standard. Please help! 🙂

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[Pisiform]

same here ... no idea

 

[Rabolisk]

The answer is D, no? I think what it's saying is that the molar mass of Y less than twice the molar mass of X, that is Y < 2X in terms of molar mass. If Y < 2X, then clearly Y < 3X, so D cannot be the molecular formula.

 

[mophead4]

Yeah, D is definitely the right answer. I guess the part I'm mostly having trouble with is how A and B are right. Take B, for instance, if its an inverse relationship with a 2:1 ratio of X to Y how can one of the answers be 1XY2? Thank you so much for all your help!

 

[ModernAlchemist]

Yeah, D is definitely the right answer. I guess the part I'm mostly having trouble with is how A and B are right. Take B, for instance, if its an inverse relationship with a 2:1 ratio of X to Y how can one of the answers be 1XY2? Thank you so much for all your help!

I think you might be overcomplicating it a little bit. If Y is less than twice as heavy as X, X could be 1 and Y could be anything less than 2. So, it is conceivable that Y could also be 1, therefore XY would result in a molecule that's components (X and Y) combine in equal mass quantities. The same method can be applied to B, but for D, what Rabolisk said:

If Y < 2X, then clearly Y < 3X, so D cannot be the molecular formula

I hope that makes sense.

 

[mophead4]

That makes sense 🙂 Thank you so much!

 

[tttgo]

The way I see it is to let y=1g/mol and x=2g/mol. Then I pretend y is the limiting reagent. For every 1 mol y, you can't have more than 2 mol x. That's pretty much what the others have said, but hope helps you understand it better.

 

[5788401]

I get how you can't have more than 2 mold of X hence answer D but how can you have a ratio of <1 for B, wouldn't that imply that there are more moles of Y than X?

 

[pm1]

I get how you can't have more than 2 mold of X hence answer D but how can you have a ratio of <1 for B, wouldn't that imply that there are more moles of Y than X?

yes, and that's okay. They said that they have the same mass quantity. For B X and Y2 have the same mass quantity implying that X is heavier than Y - you need two mols of Y in order to achieve the same mass quantity of X.

 

[5788401]

yes, and that's okay. They said that they have the same mass quantity. For B X and Y2 have the same mass quantity implying that X is heavier than Y - you need two mols of Y in order to achieve the same mass quantity of X.

ok you mean, that the molecular mass of X can be greater, equal and less than Y?
ie for A (X is equal to Y, for B, X is greater than Y, and for C, X is less than Y?...I guess the way it's written, Y is less than twice of X, I thought it had to be bigger than Y but not as big as twice but "less than twice of X" could be smaller than X too

 

[pm1]

ok you mean, that the molecular mass of X can be greater, equal and less than Y?
ie for A (X is equal to Y, for B, X is greater than Y, and for C, X is less than Y?...I guess the way it's written, Y is less than twice of X, I thought it had to be bigger than Y but not as big as twice but "less than twice of X" could be smaller than X too

yes. I know, the way they phrased it gets tricky. But the scenarios on A, B, and C holds the statement "less than twice of X" true.

 

[5788401]

yes. I know, the way they phrased it gets tricky. But the scenarios on A, B, and C holds the statement "less than twice of X" true.

cool, thanks for your help