TBR: Gen Chem chapt 1

[premedicine555]

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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

Okay, I have been reading other posts and reading the explanation, but I still can't understand it. I've tried plugging in X=2 and Y=3. How would A, B, and C not work? For A you have XY, which would be my values of 2,3... how does that work since they're suppose to have the same mass quantities? I think i'm overthinking.

 

[Amphitrite]

In this problem, the y molecule must be less than two times the x molecule. This leaves us with the ratio of the x to y molecule being greater than 2:1. So it must be at least 3:1. The only answer that satisfies that condition is D.

 

[ljc]

To elaborate further:

X and Y combine in equal mass quantities

Means you have the same amount of total mass of X as you do Y (so no matter how much each X atom weighs, or each Y atom, you have the same amount of mass total, for each)

if Y is less than twice as heavy as X

Y can be as light as it wants, but can be no heavier than 2X. this means 0 > Y > 2X.

Since Y and X combine to form the same amount of total mass, and we know Y can AT MOST compensate for 2 X's, any formula which has MORE than 2 X's per Y must be the impossible formula. This leaves D, which has a ratio of 3 X's to 1 Y.

You can't simply assign them random weights and hope it works out - your approach will not work as far as I can tell. You have to look at the ratios, because the weights can vary for each molecule.

To answer why A, B, and C are possible:

XY: Here, X and Y are equal weights, which is acceptable because 0 > Y > 2X

XY2: Here, X is twice the weight of Y, meaning Y = 0.5x. Since 0.5X > 2X, this is okay too.

X3Y2: Here, Y is 1.5 times heavier than X (2 * 1.5 = 3), and since 1.5X > 2X, this is okay as well.

Hope that helps some.

 

[rosenk]

Y<2X --> Y<3X (with X>0) --> D is not possible

For other choices:
Y<2X --> Y can be 0.5X, 1X, 1.5X
Y=1X --> XY (a)
Y=0.5X --> XY2 (b)
Y=1.5X --> 2Y=3X --> X3Y2 (c)

Hope it helps

 

[premedicine555]

ljc--THANK YOU SO MUCH!!!!! I understand. and thank you too rosenk!

 

[Erythropoietin]

This is the only question in TBR that wanted me to throw all my books out the window 🙄

 

[FutureDoc2]

This is the only question in TBR that wanted me to throw all my books out the window 🙄

My sentiments exactly!