Question

Assume the atomic mass of element X is 22.99 amu. A 13.75g sample of X combines with 21.20g of another element Y to form a compound XY. Calculate the atomic mass of Y.

Answer 1

@whpalmer4

Answer 2

If element X has an atomic mass of 22.99 amu, that means 1 mole of element X has mass 22.99 grams. We have 13.75 grams, so we can figure out what fraction of a mole of X we have. Now, we know that we have the same fraction of a mole of Y (because the compound formula is XY, no subscripts) and we know the mass of Y we have, so we should be able to work backwards to get the atomic mass of Y. As the same number of moles of Y has a greater mass than X, obviously our atomic mass value for Y should also be greater than 22.99 amu...

Answer 3

Right

Answer 4

boils down to \[\text{<moles>}*\text{<atomic mass of Y>} = 21.2\]

Answer 5

And I can tell you that you've consumed compound XY many, many times :-)

Answer 6

So the atomic mass is 21.2 then?

Answer 7

no. 21.2 is the mass of element Y in (however many moles)

Answer 8

if it turned out that we had exactly 1 mole of element X, then 21.2 g would be the atomic mass of element Y (because we would also have 1 mole of element Y). however, we don't have 1 mole of element X.

\[\text{<atomic mass>}*\text{<number of moles>} = \text{<mass in grams>}\]

We know the mass in grams, and we can find the number of moles because we know that it combines 1:1 with element X and we know the number of moles of element X. Plug in those numbers and find the atomic mass.

Answer 9

How do you know you don't have 1 mole of element X?

Answer 10

because the formula we were given was XY. 1 atom X, 1 atom Y

besides, why would you assume that you had exactly 1 mole of Y when you had some random amount of X, definitely not 1 mole? That doesn't seem like a good way to get a correct answer, does it?

Answer 11

It doesn't.

Answer 12

So for this , am I using the information from X or Y to find the Y's atomic mass? I know I would use the grams for the Y but what of the X's am I using?

Answer 13

Okay, are we clear on the 1:1 ratio of moles of X and Y?

Answer 14

No. A bit confused right now.

Answer 15

Well, think of the formula XY as a recipe for making the compound XY. It could have been written
\[\text{X}_1\text{Y}_1\]but one doesn't bother specifying that there is only 1 atom, it is taken for granted, and only if the formula involves more than 1 (for example, \(\text{H}_2\text{O}\)) is the subscript needed.

So, no matter how much of compound XY we have, we know that we have the same number of moles of X as we do moles of Y.

Answer 16

Is that clear?

Answer 17

Okay. I was mixing up atom with mole.

Answer 18

I hope it is, because I don't have any other way to explain it...

We know that however many moles of X we have in our compound, we have the same number of moles of Y.

We also know that 1 mole of an element with atomic mass \(n\) amu has a mass of \(n\) grams (and \(6.022*10^{23}\) atoms).

We know both the atomic mass and the mass of element X, so from that we can calculate the number of moles of element X present. Just divide the mass of X by the atomic mass of X (expressed in grams) to get the number of moles or the fraction thereof.

Answer 19

How many moles of element X are present, given that we have 13.75 grams and the atomic mass is 22.99 amu?

Answer 20

.598

Answer 21

is the moles of X

Answer 22

If you take (13.75g)/(22.99 amu)

Answer 23

That's correct. So how many moles of Y do we have?

Answer 24

same number!

Answer 25

Since it's 1 to 1?

Answer 26

exactly. so, we know the number of moles of Y, and we know the mass of Y. All that remains to be determined is the atomic mass of Y.

Answer 27

So divide (21.20)/(.598) = 35.45

Answer 28

Yep! Now if you look at your periodic table, you should be able to figure out which two elements X and Y are, and what compound XY is, and why I am so sure that you regularly consume it :-)

Answer 29

It's Chlorine with sodium right?

Answer 30

X is sodium, Y is chlorine, yes. And the two together make plain old salt.

Answer 31

Yep. Thanks for helping me with that

Answer 32

you're welcome! hope that helps you with the rest of your work, I'm out of here for the rest of the night.

Answer 33

Have a good night.