# Finding element X in an unknown compound

1. Jun 9, 2015

### Saracen Rue

1. The problem statement, all variables and given/known data
An unknown compound weighs 1.023 kg. It is known that element X composes 28.3% of the compound. Given that the molar mass is 240.5 g/mol, find element X.

2. Relevant equations
mole = mass / atomic weight
(n = m / Ar)

mole = mass / molecular weight
(n = m / Mr)

mole = number of particles / Avogadro's number
(n = P / NA)

3. The attempt at a solution
I tried solving using the first equation, but I don't know them mass, Ar or mole of element X. I solved for the mole of the substance using the second equation, but I don't know how to find the unknown element from there. I don't think the last equation is applicable in this particular case, but I included it just in case.

2. Jun 9, 2015

### Staff: Mentor

Neither of the equations you listed is applicable here, answer is much more trivial.

Pretty basic math: you are being told x grams compose 28.3% of 240.5 g. How many grams is x?

Not that I know what the final answer is. Unless you did some mistake copying numbers, there is no element that fits.

3. Jun 9, 2015

### Saracen Rue

I think I've worked out the problem. 28.3% of 240.5 g isn't actually giving the Ar of the element - it's giving the Ar of the element x the number of said element in the compound. I can't work out how I could find just the Ar on its own, without the number of the element factoring into the problem.

4. Jun 9, 2015

### Staff: Mentor

The only valid approach is to guess - assume there is a single atom, or two atoms, or three atoms per molecule and so on. Sadly, none of the numbers you will get gives a reasonable answer.

5. Jun 9, 2015

### epenguin

And then the first sentence of all makes no sense, and on any near guess as to sense it is irrelevant... I don't think we should try to guess further.