This question is driving me CRAZY!

[Lunasly]

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So I am on the topic of molecular and empirical formulas:

Q: 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 the following molecular formulas is NOT possible?

A) XY
B) XY2
C) X3Y2
D) X3Y

Answer is D.

So because Y is a little less than twice as heavy as X, then that means there must be a little less than a 2:1 (X:Y) since the question says they combine in equal masses. Can someone explain to me why A,B,C are wrong and why D is right?

Thanks.

 

[rjosh33]

So I am on the topic of molecular and empirical formulas:

Q: 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 the following molecular formulas is NOT possible?

A) XY
B) XY2
C) X3Y2
D) X3Y

Answer is D.

So because Y is a little less than twice as heavy as X, then that means there must be a little less than a 2:1 (X:Y) since the question says they combine in equal masses. Can someone explain to me why A,B,C are wrong and why D is right?

Thanks.

This question seems a little ridiculous, but the "less than twice as heavy" part is key, obviously.

a.) A 1:1 weight ratio of Y to X is less than twice as heavy.
b.) XY2 indicates X is twice as heavy as Y, meaning, of course, Y is less than twice as heavy as X.
c.) For X3Y2, Y is 1.5 times as heavy as X, which is less than twice.

But for X3Y to hold true, Y would have to be roughly three times as heavy as X, making it the violator of their stipulation.

 

[cloak25]

basically, you can't have more than 2X's per Y.
for XY ==> you have 1X per 1Y
for XY2 ==> 1X per 2Y = 0.5X per Y
for X3Y2 ==> 3X per 2Y = 1.5X per Y
for X3Y ==> 3X per Y

all the choices meets the requirements except D.

 

[Lunasly]

What does it mean then when it says they combine in equal masses?

 

[ingramw1202]

What does it mean then when it says they combine in equal masses?

It means that 1 g of X will combine with 1 g of Y to yield 1 mole of XnYz, where n is number of X atoms and z is the number of y atoms. So if Y is less than twice as heavy as X then the molar mass of MWy < 2 MW x.
A. XY. If this is the case, then X1Y1. If Y is less than twice as heavy as X, MWy can be any number less than 2MWx. So X1Y1 satisfies this requirement because if you had 1 g X combining with 1 g of Y, then X1Y1 is possible when the two have the same MW..which satisfies the requirement.
B. XY2. In this case, 1 g X combines with 1 g Y to make X1Y2. If MWy< 2MWx, then 2 mol of Y are in 1 g of Yand 1 mol of X is in 1 g X ( this is a ratio). So for every 1 mol Y, the weight of Y compared to X is exactly half, which satisfies the requirement.
C. X3Y2. So if you combine 1 g of X and 1 g of Y, you end up with X3Y2. This means that in 1 g X, there are 3 mols of X, and in 1 g of Y, there are 2 mol of Y. So the MWy=3/2 MwX, which satisfies the requirement.
D. X3Y1. 1 g Y and 1 g X yields X3Y1. So 1 g X is 3 moles of X and 1 g Y is 1 mol of Y. THis means that MWy=3MWx, which does not meet the requirement.

 

[LoLCareerGoals]

This is an algebra question nothing more. Forget chemistry aspect of it.
Equal mass combo means in the compound M(all X) = M(all Y).
Less then twice as heavy merely means M(of one Y) <= 2*M(of one X), THIS CAN MEAN that Y is lighter then X or heavier or same.
A) M(of Y) = M(of X), satisfies (1<=2)
B) M(of Y) = 0.5 M(of X), satisfies (0.5<=2)
C) M(of Y) = 1.5*M(of X), satisfies (1.5<=2)
D) M(of Y) = 3*M(of X), ALERT ALERT!!! (3 <=2? NO. )

Oh can you not post the correct asnwer next time? Give other posters a chance to think/practice and possibly make mistakes they could learn from.

 

[gettheleadout]

Oh can you not post the correct asnwer next time? Give other posters a chance to think/practice and possibly make mistakes they could learn from.

Or post in it white text.