# Basic Stoichio with a missing element? Please help :(!?

a compound is composed of element X and hydrogen. Analysis shows the compound to be 80% X by mass with three times as many hydrogen atoms as X atoms per molecule. Which element is element X?

the answer is carbon

### 2 Answers

- AshleyLv 68 years agoFavorite Answer
XH₃ means a mass of 3. so 3 is 20% of some number. 300/20 = 15, so the whole mass is 15. That means the mass of X is 12. That would be carbon

- Anonymous8 years ago
So your formula is XH3, right?

It's got to be a group 3 or 5 element, because those take 3 bonds. NH3, ammonia, BH3 boron trihydride or AlH3, aluminum trihydride.

N is 14g/mol. H 1g/mol

NH3 is 17g/mol.

14/17 = 0.82 (82% mass) while 3/17 = 0.18 (18% mass)

BH3

B 10.8g/mol so BH3 = 13.8g/mol

10.8/13.8 = 78.3% B, 11.7% H

AlH3

Al 27g/mol so AlH3 30g/mol

27/30 = 90% Al, 10% H

Can't be this one, % too high

Gotta be N or B... which is a choice on your test? Pick that one.

Close enough for govt work! :)

Hope this helps! :)

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